Most import engines use a vented thermostat (an opening at the top-install to face top-that allows air pockets to be released. Sounds like you have an air pocket behind the thermostat. Have you tried a genuine MOPAR thermostat?
The thermostat has to be in there to properly regulate the coolant flow. The thermostat opening/closing will actually vary with continuing coolant temperatue change. So it has to be a quality piece.