Well, it sounds like the clutch is OK. It just runs at the engine speed when it's engaged and "slips" when it's not. I can hear the electric fan start on my car.

I would start looking at the cooling system. What sort of flush was done, was the engine run with an acid flush, or was it just rinsed out? If you search you will find some info on performing a citric acid flush, that might be the next step if you are confident the thermostat is good.