1991 Plymouth Acclaim Temperature Gauge Fluctuating?

I have a 1991 Plymouth Acclaim that seems to heat up, then cool back down. The temperature gauge goes all the way to hot, then after a few seconds it goes back to normal temperature. The water pump just got replaced, and the coolant is full. It doesn't seem to run any different when the gauge is reading hot, but I don't want to drive it like this and cause any damage. Any idea what could be wrong?