2000 sunfire heating problem

00 sunfire was overheating so I changed the thermostate and now it barely heats up when driving and at a stop it heats to normal temp then goes down once it stsrts moving. It has the 2.2L in it

Might be a faulty coolant Temp sensor.

that's the way they run, my 2.2L (when I had it) ran cold as well unless it sat. I changed the thermostat to try to correct the issue, but it still ran the same way, so I didn't mess with it any more.

If you change the thermostat, be sure to use a GM part, as the aftermarket t-stats have a weak spring, which lets it open even though the vehicle is not up to temperature. My wife's car had the same problem. Hope this helps.