Temp Gauge is "Floating"?

So, I have a 96 Buick Regal...I have just replaced the Water Pump and put a new Thermostat in it. I also have put new coolant mix in it..yet the gauge still floats? (It'll go up to red then back down &do that over &over &over...yet NEVER sticks.) I have no idea how to make this stop...or what else could be wrong? Help!