Automatic Transmission Cooler problems of the 2008 GMC Envoy

I purchased a 2008 Envoy denali 7/08 during the middle of November while drive the temp gage drop to the lowest point and started cutting off hesitating to start. I then contact on-star they stated it was the cooling system and should be taken to the dealer on 10/19/09 I went and was told it was the temp senor which was replace that Tuesday. Thursday the same issue accord again I notified on-star they stated this time it was the transmission Friday I went to the deal to be told they could not work on the car until Monday. As I ws speaking to a tech they stated it sounds like the trans and the mgr approached and stated I am handling this and directed me toward the desk and stated he had a rental for me and arranged a ride for me. I truly believe there is something wrong with this car and to make this worse my navigation system has failed me twice as well.