Help... Does anyone know what would be causing this:

1994 Galant LS

The temp gauge reads 3/4 of the way to Hot. I changed the thermostat and the temp switch. When I started the car, the gauge went from below Cold to 1/4 up in a flash. I drove regular streets and the highway w/ the ac and heat off. It gradually went to normal for the car, which is just below 1/2 way up. When I got back off the highway I turned on the AC. the gauge jump within a half second from normal to 3/4 hot. Stayed there. I turned the ac off and the heat on. No change. I then turned everything off and drove for 2 mins before it jumped back down to normal. Not a gradual change, but a jump.

The fans run, I assume normal.

I just can't figure out what's going on w/ the gauge. Is the car running that hot? Is there something i'm missing that I need to check or replace? I can't find my Haynes book for the car.

Any suggestions?
Thank you.