2010 4.0 AT Coupe, thermostat housing sprung a leak at approx 100,000 so I ordered a metal replacement which included a thermostat and temp sensor. Installed, got the air out of the system, car ran fine for a couple of weeks. Heater was blowing very well, cold to hot. Now it'll run a few minutes, temperature warning light comes on, temp shoots up (not gradually but instantly) for C to the mid point on the gauge. The temp gauge will then drop back to C very quickly, then temp waring light comes back on, and the process repeats. At this point, the heater is blowing luke warm. I've parked it until I can figure this out. My first guess is they put a cheap thermostat in the housing to keep the price down.