1991 Plymouth Acclaim Temperature goes up and down

I have a 1991 Plymouth Acclaim. The temperature gauge goes almost all the way to "H" and then goes back down to "C", then it will settle in the middle for about 5 to 10 minutes and then go back up to "H" and then right before it hits "H" it goes back down "C" then comes up a little. This continues to go on as I drive the car. I changed the thermostat and it still does it. This use to never happen.