Make sure they check out the FAN or the module that drives it..... if your temp gauge is showing hot and the coolant level is good then the fan may not be working right.. there have been several threads on fan failures..

The fluid sensor could be bad, but that wouldn't show the hot engine. Also check the connection of the sensor. Unplug it and re-connect to make sure it is on good.

Did you thank them for the new alternator??