Car overheats when using ac.

The idle fluctuation could be the A/C compressor kicking in and out. If that fluctuation goes away when the A/C is turned off, then you would need to investigate why the A/C is not on more steady.

The increased heat from the use of the A/C is normal. But you need to find out why or if the cooling fan is not running (or is it running?). The cooling fan is suppose to compensate for using the A/C.

I wrote earlier about there being Relays for the electric cooling fan. The Relay is activated by a temperature sensor. About the old cooling fan--did you test it to see if it worked off the car? If you did not need a new electric cooling fan you still need to fix the Relay or the temperature sensor.

Now replace the thermostat. The other part on some vehicles (usually not on transverse mount engines) is the mechanical cooling fan clutch. This is checked by measuring rpm of the clutch blades. It takes a special adjustable timing light to do this. The clutch is fluid filled and adjusts its grip by the air temperature around it. The amount of slippage is why rpm is measured. It is suppose to slip most in the cold and then lock in place in warmer temperatures.

Also I would recommend "aboutautomobile.com" for reported problems. Also web search overheating problems for your model. The idle fluctuation could be the A/C compressor kicking in and out. If that fluctuation goes away when the A/C is turned off, then you would need to investigate why the A/C is not on more steady. The increased heat from the use of the A/C is normal. But you need to find out why or if the cooling fan is not running (or is it running?). The cooling fan is suppose to compensate for using the A/C. I wrote earlier about there being Relays for the electric cooling fan. The Relay is activated by a temperature sensor. About the old cooling fan--did you test it to see if it worked off the car? If you did not need a new electric cooling fan you still need to fix the Relay or the temperature sensor. Now replace the thermostat. The other part on some vehicles (usually not on transverse mount engines) is the mechanical cooling fan clutch. This is checked by measuring rpm of the clutch blades. It takes a special adjustable timing light to do this. The clutch is fluid filled and adjusts its grip by the air temperature around it. The amount of slippage is why rpm is measured. It is suppose to slip most in the cold and then lock in place in warmer temperatures. Also I would recommend "aboutautomobile.com" for reported problems. Also web search overheating problems for your model.