Oldsmobile Alero

My temperature gauge has been acting up. It stays above the middle and goes up and down while I am driving. It will go all the way to the bottom of the red and then just drop right back to the middle after a few seconds and then do it all over again. I took it to 2 different garages. The first told me there was an air pocket in the thermostat that they cleaned out. I don't even think they looked at it though. I took it to another one who changed my thermostat and did something to the gauge and it is still doing it. They said it could be my cluster going bad. Any advice on what it could be because it's driving me nuts!