When air conditioning is on and i'm in stationary traffic, the engine tempreture sores to 3/4 on temp gauge. As soon as I turn a/c off temp drops back to half way on gauge, otherwise even when moving and a/c on temp still sits over half way.

Turn off warm engine .. Fan should stop turning in 3 seconds!.. Need a helper to test.

when a temp gauge drops to cold ,it indicates a problem in the cooling system
temp gauge sender units need water to activate the resistance that makes the gauge read so a falling gauge indicates no water around the sender unit --- air lock in the system--faulty wire connection to the gauge sender unit
the ac unit is not connected to the temp gauge so there may be another separate problem there
the temp sensor is an entirely different unit to the temp gauge sender unit and the short term working would be from an altered coolant level from changing that sensor --air lock in system
the temp sensor connects to the ECM to activate the fans and monitor engine operating temp so as to keep the air/fuel mixture exact for the engine conditions
the temp sender unit just makes the temp gauge read
a faulty thermostat will either be stuck open and the engine temp will take a long time to rise or it will be stuck closed and that will cause an overheat very quickly

If you have a viscous fan hub then have it replaced as it is not drawing air through the radiator at idle or slow speed (crawling traffic). Check that are no air locks in the system as they will cause heating problems.

Its fine, temp needle going up shows that engine is warming up and as soon as the radiator fan starts, temp of coolant drop and this is shown by the temp gauge needle. In most cars (even mine) it stays in middle most of the time unless you are in a traffic with AC on, it will show up more temp. What you can do is get the radiator flushed and coolant changed. Also check the water pump. To do so when engine is cold (in first start in morning), open the radiator cap and ask someone to rev the engine. If the coolant level moves up and down then water pump is also fine.

temp sending unit bad, or wires shorted to ground.

Have your radiator flow checked you may have a clogged radiator.

Sounds like faulty engine temp sensor. When it drops to cold, the engine control unit picks this up and alters the fuel mixture accordingly. The ECU will be making it a very rich fuel mixture for cold starting, hence very poor running when engine is hot.

TL

I had the same problem or more so that my temp would run cold for miles and miles then go to normal and drop. It was my glow plugs. They needed to be changed. My boyfriend changed them himself in 10 minutes after I had been quoted 2 HOURS for labour!!

Check the auxiliary cooling fan for operation. As the engine temp rises, it should kick on to bring the temp down. U may have a bad motor on the fan, or more likely a bad fan switch.

It could be very well your water pump from the sounds of it. When was the last time you had that check or replaced? That would be my next guess to the problem...what's your mileage on the car?