I am confused, is there any other reason that the heat would not work in the cabin. It blows sporatically. I know the heater coil is good, there is plenty of coolant, could there be anything other than the thermostat that could be amiss? I will call the shop that replaced it and see where it was purchased.
Well, the best way to find out if the engine is at the correct operating temperature is to either place a thermometerin the radiator neck to take a direct engine coolant temperature measurement, or use a non-contact thermometer (IR temp gun) to take an engine temperature measurement at the thermostat housing. You can plug a scan tool into the car's data connector and read engine temperature on the scan tool as reported by the coolant temperature sensor, although sometimes this is not totally accurate as coolant temp sensors are not quite as accurate.
My concern here is that you stated the engine temperature gauge is not reading as high as it used to/ where it was normally before the problems started. This needs to be investigated to find out if the engine temperature actually IS below normal. If the engine temperature is at a normal 195 degrees then there is a problem affecting only the heater operation, such as a restricted heater core, air trapped in the heater core, or an internal temperature blend door problem inside the heater case.