Last year my wifes car developed a problem with the heater and it is starting again. The heater seems to only work when the engine RPM is ABOVE idle. IE.. If sitting at a stop light, the air coming out of the vents is cold, but as soon as you press on the gas, even just a few hundred RPM's the air starts coming out hot again. On one occation, the engine temp started to rise above normal temp and when she pressed on the gas it came back down. I purchased a new thermostat, but I have heard alot of issues about the "t-stat housing breaking" becuase it is plastic.. Any information would be helpfull.