Overheating when stopped. New thermostat, new head gaskets. Antifreeze will be boiling and spill out the reservoir no matter what antifreeze level is.

Pam are you sure it's boiling? If the Antifreeze is boiling I'd think you'd see steam coming out of your reservoir as the water expands into the reservoir, but you didn't say you see anything like that.
Of course engines get hot, that's why we have the cooling systems but it doesn't sound like it's "boiling" to me with no signs of visible steam unless your antifreeze level is really low?

If it's really boiling, and you antifreeze level is good, then your reservoir should be getting filled up with expanded hot water that goes into it to catch it when it expands.

If the water isn't going into the reservoir, then maybe your antifreeze level in the radiator is too low and you need to add antifreeze?
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Best to fill the radiator with the engine cooled down. If you ever have to open the radiator when it's hot, NEVER open it with your bare hand as you could get scolded with boiling water. Instead use a big thick towel to remove the radiator cap off a hot engine. Again, if you can, just wait until the engine cools then open the radiator cap.

With the engine cool, you can fill the radiator with a 50/50 antifreeze mix. The 50/50 mix come premixed 50 percent water, and 50 percent antifreeze. So you can throw that stuff straight into the radiator.

Non mixed antifreeze needs to be mixed 50/50. Some people use 50 percent distilled water when they mix it because distilled water doesn't have the minerals in it that may get stuck in your radiator. That's being a bit picky about it though. Tap water works fine in a pinch. The other 50 percent is antifreeze. You can mix it right in the radiator too, put in a measured amount of the antifreeze, followed by the same measured amount of water. The water pump will mix it all together when you run the engine.

Put the cap back on and run the engine and if/when the water "boils" it should go into your reservoir. If it does boil in go in the reservoir then yes...your engine is getting hot and the antifreeze is boiling.

When the antifreeze boils, it's probably because your thermostat is stuck closed. That means the thermostat isn't letting water circulate through the radiator to get cooled. Thermostats are cheap...no more than $30 I'd say for most cars. Changing them is pretty easy too as they are usually right at the top of the engine. A mechanic may charge $100 to change it....but it's an easy job on most vehicles requiring just a couple sockets/socket wrench.

You have a blown head gasket or a cracked head..more than likely just a blown head gasket..you need to have someone run a combustion gas test on the radiator..you are getting raw exhaust pushed back in to the radiator causing it to superheat

Do a combustion leak test at the coolant cap for
exhaust gases

Need to be sure your not loosing coolant & have no
air in the system

You have a head gasket problem with the engine generating abnormal pressure, and sending all the antifreeze to the reservoir, thereby leaving the engine running dry; it is dangerous for the water pump to operate with no water.
Drill 3 11/64 or 3/16" holes in the thermostat side facing the engine, making sure not to touch the opening mechanism; this will keep antifreeze always running thereby, compensating for the abnormal pressure regenated by head gasket leak(s).
Your thermostat was never your problem.
In cold weather, 5 degress celsius or under, the thermostat takes longer to open and more antifreeze will be sent to the reservoir leading to an overheating.. Partially cover the radiator with a sheet metal piece to make the thermostat open faster and stay open, as the antifreeze is not being cold that much by cold air. You're fooling the cooling system into warm weather driving.
Don't waste your money buying radiator stop leak or liquids to repair head gaskets; this will only corrode and make your radiator to eventually leak.

You have an air bubble at the top of the engine.
The antifreeze spewing out of the overflow happens due to your perfectly functional radiator cap
passing the overheated coolant/air mixture out of the system into the reservior (in an attempt
to relieve overpressurization of the cooling system)
The sudden nature of your overheating comes from the boiling of the inadequate amount of
coolant/antifreeze in your engine, since it cant properly cool the engine, so it boils (overheats quickly), and pressure rises immediately within the cooling system.
The Fix is free: locate and open the (air) bleeder port at the top of the engine, and add the usual
50% coolant/50% water mixture to the radiator until the radiator is full to the level indicated in
your owner's manual (or the "Full" line). Then close the bleeder port and replace the rad (radiator)
cap.
The air bleeder port is normally located near the thermostat, and should be opened each time
that coolant is added to the radiator. The air bleeder port need not be opened when adding
50/50 coolant to the reservior, since the rad cap is going to be closed while adding coolant
to this part of the cooling system.
Warning: driving beyond the point where your engine temperature gauge indicates overheating will
destroy the car's engine - it'll seize up due to overheating, and the only fix for a seized/burned-up
engine is replacement (of the engine). Seizing an engine is really easy with an air bubble in the
top of the engine - you could easily burn up/warp the head, necessitating replacement of the head
and head gasket.

I would replace the rad, you've done everything else.

why is the engine overheating. 1. Does the coolant have the correct 50/50 mixture.can be tested with a antifreeze tester 2.does the radiator get hot.if not restricted coolant flow which could be thermostat not working proper. 3.Cylinder head gasket not sealing proper and allowing combustion chamber pressure to leak into coolant chambers

the engine is overheating or u have a defective radiator cap

it sounds like a possible head gasket problem,you can get the fluctuation from combustion gases getting pumped into your cooling system and creating gas pockets,i would suggest taking it to a shop and have them do a block test to look for combustion in the cooling system to confirm the problem,that way you will know for sure,if it comes up negative they can also perform a flow test to make sure there is no restriction in the cooling system such as a plugged radiator or a bad thermostat,hope this sheds a little light on it

One of the common things that gets missed at times is the radiator is down in cooling antifreeze. The car will act as normal during the warm up cycle and then the temperature guage will rise and then it will slowly come back down as the thermostat opens and/or the electric cooling fans come on. In actuality the engine is overheating, but it is not boiling over. The level of abtifreeze is at just the right level of lowness that you experience this phenomenon. After the engine cools off , (DO NOT REMOVE WHEN HOT - YOU CAN GET SCALDED WITH BOILING WATER/ANTIFREEZE)), remove the radiator cap and check the level within the radiator and also the antifreeze reservoir level. You can buy a 50-50 mix of antifreeze and water that is ready to put into the radiator. If the level in both the radiator and reservoir are correct, you may have a bad thermostat or some other problem.
Bob - 2003 Nissan Sentra SE-R SPEC V