96 Buick Regal's Temp Gauge is floating?

Okay so I just changed the water pump 2 days ago...the old one was leaking BAD!...I changed the thermostat &put new 50/50 antifreeze in it. Now the temp gauge is floating..like it will go all the way to red..then back down. It never sticks tho so I was wondering what could be wrong?? My dad thinks it has something to do with the intake gasket??