Car Accelerates On Its Own problems of the 2008 GMC Acadia

Vehicle experiencing bad transmission problems when shifting between 3rd and 5th gears. As accellerating, the vehicle begins to shift, but begins to idle and rpm's rise extremely high. Vechile begins to slow down and finally jerks into the next gear causing rapid acceleration. This issue almost caused me to run into a car in front of me after pulling off from a stoplight. It is extremely scary to continue to drive the car, but with this being my only vehicle and I must work, I am forced to drive daily. Warranty just expised 5 months ago in March and I cannot afford to fix the problem (March 24, 2013, 5 year/100,000 mi. Powertrain). Vehicle was purchased March 24, 2008.

I noticed that when driving on an expressway and would attempt to pass a slower moving vehicle, as I would move into the passing lane and attempt to accelerate, the car would hesitate for a few seconds, lose speed, then suddenly downshift and accelerate at an unexpected rate. I also noticed that the car would unexpectedly accelerate or decelerate under normal driving conditions. I bought the vehicle to the dealer for repair, they responded that they could find nothing wrong, but the what I was experiencing was characteristic of the 6-speed transmission on the vehicle. The problem continued and I found it dangerous. I returned the vehicle and the dealer's remedy was to "reset the transmission computer. " during this time, I also noticed that the "service stabilitrac and traction control" warning would come on. Onstar recommended I shut off the engine to reset the system. I found this only happened when I first used the car in the mornings to go to work. Again, the dealer said there was nothing wrong and the computers were showing no codes -- they also stated that they could not duplicate the problem. I got the car back from the dealer and it happened again - at 7:00 am. I called onstar and they gave me two codes. I bought the car back to the dealer and gave him the codes. It now seems that there were two defective sensors controlling the stabilitrac / traction control system and how they communicate with the transmission. I was told that gm is changing manufacturers on the sensors. I have not had use of the car since the beginning of June and am still waiting for gm to produce these parts. In the meantime, unexpected acceleration, deceleration, and gear shifting was occurring under all driving situations causing -- in my mind -- a danger to myself, my passengers, and others. The dealer has had the car on five different occasions since June 8. This last service they have had the car since July 20.