I'm one grumpy gal. I just forked out 1300 dollars for the installation of a new head gasket set...and i just got my car out of the shop and it overheated. So, I returned again to the shop and they installed a new thermostat. I drive the car again and an hour later the temperature gauge is in the red, and fluctuates, but no smoke is coming out...they can't figure out the problem. Any ideas?

Could be a number of things, waterpump, not enough coolant, radiator, blockage. First thing to do is check the level of coolant, second is to pressure test it to see if it's leaking. What kind of weather do you have there temperature wise? Often if a waterpump is going bad, the temp needle will move and fluctuate with RPM. Or if you're stuck in traffic, it could be the cooling fan.

And make sure it's really getting hot, like they didnt just forget to plug something in.