has the system been bled of air? if not that can be a problem.
are you loosing coolant, are you checking it regularly after these shop repairs?
the heat, could be bad blend door actuators. to test this you'll want to grab the hoses that lead into the center of the fire wall from under the alternator, and see if they are both hot. if both are hot, bad blend door actuators. one hot, one cold. or both cold, is air in the core and lines.
if your loosing coolant, being your a N/A engine, the upper intake gaskets like to fail, the engine then eats coolant, as well as dumping it into the engine oil, then the engines goes boom.
so also check your oil, by just pulling the dip stick out, make sure its not milk shake looking. or foamy. just oil.
