My car has been stalling on cold starts and won't shift into overdrive consistently (AUTO trans). It will stay in overdrive and then randomly drop into 3rd gear and then like 45 minutes later shift back into over drive, while highway driving. Today I realized that the coolant temp was staying around 160-180 which seems kind of low and the heater wasnt throwing off to much heat. The transmission was rebuilt less than a year ago and it still shifts into overdrive though not all the time, so I don't think its going, at least I hope not. Any ways before spending alot of $ I figured I should replace the coolant temp sensor and the thermostat. Where is the thermostat located? Is it difficult to replace? Any help or suggestions would be appreciated.