OVERHEATING I HAD THE INTAKE MANIFOLD GASKET REPLACE, CHANGE THE WATER PUMP AND THERMASTATE AND THE CAR IS STILL OVERHEATING. I BLEED THE COOLINT LINE. HOW LONG DO YOU HAVE TO BLEED THE LINE.

Coolant leaks,When the intake manifold gasket cracks or blows out, coolant will leak out of the intake manifold. You may see puddles underneath your car or notice smoke or steam coming off of the engine and exhaust from the coolant dripping onto it and then burning.Overheating, If the intake manifold gasket fails and allows coolant to leak out into the engine, your car will begin to overheat. If your car frequently runs low on coolant or begins to overheat, check the intake manifold gasket.Running poorly, When the intake manifold gasket falters, it affects the way the motor handles its emission gases. The change in pressure can affect the vacuum in the intake as well as how smoothly your car runs. Mainly, it will idle roughly and may hesitate, cough or sputter.

you have a plugged radiator or heater line to back heater unit not allowing coolant to flow

The intake manifold has a water passage that takes water from one side of the engine and moves it to the other side in a V6 engine. Your engine has a plastic tube that does this job. That tube can leak which leads to water and pressure loss and that leads to overheating. Ask him if he is referring to the water crossover tube in the intake manifold. If that is what he is referring to the that is a 6 hour replacement job. You should have the water pump and belts replaced at the same time along with upper and lower radiator hoses. That price is a bit high but not that out of line.

Sounds like you still have a cooling system leak issue like a cracked cylinder head.

Drain radiator. Remove upper rad hose from thermostat housing. Disconnect air intake hose from throttle body. Remove throttle body from upper intake manifold, disconnecting the 2 rubber hoses to the heater line. You may also need to disconnect the fuel line. Remove the exhaust crossover heat shield and loosen the crossover pipe from both exhaust manifolds but don't remove crossover completely. You should have access to the 2 thermostat housing bolts, remove upper bolt but just loosen the lower one about a half inch. Pull the housing out and up from the intake manifold to access the thermostat. Clean the mating surfaces with a gasket remover. replace thermostat, apply a light coating of rtv sealant to both sides of the new gasket. Replace thermostat housing with the gasket to the intake manifold. Apply some rtv to the bolt threads before reattaching housing. Tighten bolts snugly but do not over tighten. Tighten exhaust crossover and reattach the heat shield. Install throttle body attaching the 2 little hoses at the bottom, its a good time to replace these hoses and clamps. Depending on the condition of the throttle body gasket you may need to replace it also. Reconnect the fuel line (if disconnected) Reconnect air intake hose to throttle body. Reattach the upper rad hose to housing. Fill rad with coolant 50/50 mix with water (make sure to use the same type antifreeze that is in the system, they say you can mix green with red but I wouldn't recommend) Leave rad cap off start the car and let it warm up until you can see the coolant circulating and ad coolant as necessary until full. Now there is going to be some air in the system that you need to bleed out. There are 2 bleeders on this system, 1 is on the top of the thermostat housing, and the other is on the metal heater line on top of the water pump (the one that goes across the front of the engine and around under the throttle body that the two little hoses attach to) Bleed the one on the housing first (its the lower one) open it until fluid comes out then tighten it. Then move to the 1 on the heater line(above water pump) loosen it until fluid comes out the re-tighten. Fill recovery tank to fill line replace the rad cap and take it for a ride, watching the temp gage closely. If the temp goes higher than normal than there is probably more air in the system. you'll have to bleed it again.
This is a very labor intensive procedure and I do recommend buying or checking a service manual out from your local library. I've done these in the past and to be honest I just have the dealer do it now. A system flush and thermostat replacement was around $230 the last time I had it done, but it comes with a warranty. And you wont have to rent or buy a fuel line disconnect tool if you end up having to disconnect it. These are the hardest thermostats I've ever replaced.

If you have a 3800 motor check your intake manifold gasket these are the symptoms that my car exhibited untill the motor locked up because it had filled the cylinders with water.
The dex cool deteriorates the gasket and the plastic intake manifold.
Replace the intake manifold and gasket, flush the dex cool out of the motor and replace with the regular green coolant.

Since problem developed after head gasket replaced. I would look at head gasket. Try re-torque head bolts. Was correct gasket size installed? (due to wear and tear, sometimes OEM gasket is no longer correct).

If its the 3.1L it could be your intake manifold gasket , the material used for the gasket does not react well with GM's Dex-Cool antifreeze. The leak may not be apparent at first, but I would pressure test the cooling system. If you look at the front of the motor by the belts , try to peek down at the intake manifold and check for slight seapage to an obvious leak.

If its either of the Quad-4's they were notorious for heads warping or cracking.

1 Inspect Cooling System Mix Coolant level low or flow is restricted. 2 Inspect Belt Incorrectly routed, adjusted, tensioned, missing, or worn water pump belt(s). 3 Inspect Oil Pan Gasket - Performance Ruptured, cracked or leaking radiator hose. 4 Inspect Radiator Cap Worn or damaged radiator cap 5 Inspect Thermostat Thermostat stuck closed 6 Inspect Fan Blade Broken, missing, or defective fan blade(s). 8 Inspect Water Pump Damaged, worn or leaking water pump. 9 Inspect Intake Manifold Plenum - Perform Leaking water pump gasket. 10 Inspect Cooling Fan Control Faulty cooling fan control or circuit. 11 Inspect Cooling Fan Switch - Radiator Faulty radiator cooling fan switch or circuit. 12 Inspect Engine Temperature Sensor Faulty engine temperature sensor or circuit. 13 Inspect Temperature Switch Damaged or faulty temperature switch or temperature switch circuit. 14 Inspect Fan Clutch Worn, loose or faulty fan clutch. 15 Inspect Ported Vacuum Switch Damaged, leaking, or faulty ported vacuum switch. 16 Inspect Radiator Obstructed radiator core or radiator cooling fins. 17 Inspect Head Gasket - Performance Head gasket leaking coolant into cylinders

It sounds like you have air pockets in your cooling system. Those engines have bleed valves strategically placed about the engine. At the radiator filler neck it looks full but in reality you're low on coolant. They're a bugger trying to eliminate the air pockets. That's what it sounds like with the informaion you gave. Good luck!