Engine temperature fluctuates in Dodge Spirit

I have a friend with a 93 Dodge Spirit (Yes, I know it's not an A-body) with a 4 banger in it. The temperature gauge fluctuates while driving. It goes from 3/4 hot to almost cold. I have changed the thermostat, no change. There seems to be plenty of flow in looking in the radiator and the car is not overheating. It has a new head gasket too, there was a bad spot between #1 and #2 cylinder. It runs perfect now and does not seem to be running hot.