2.4 liter 2001 Galant runs fine at hwy speeds, but in driveway with idle the temp needle goes into the red. Turning the heater on drops the temp guage.

I changed the thermostat and pressure tested the radiator (fine).

Not sure what is wrong.

Hmmm, thats tough. I would assume that you're not overheating at highway speeds because there is more airflow to the radiatior. Do you have enough anti freeze and is your water to anti freeze ratio in check? You may need to put some more anti freeze in because if you have too much water it may have trouble keeping cool at idle with no wind flow coming into it and it could keep cool at highway speeds because it has more air flow. Good luck.

check ur radiator fan if is not working check relays that make u car overheat at idle and check all u belts