My parents have a 2005 Chevy Uplander with the 3.5 V6 / 4 speed auto (4T65E). For the past 2 years or so when driving on the highway for MORE than 1.5 hours the car will start to hesitate / surge followed shortly after by what seems to be the torque converter or 4th gear letting go. The rpms will go up to 3000 at 70 MPH (normally around 2300 or so). The trans will stay in that gear (assuming 3rd) for about 10 minutes then shift back down and act normal. This process will continue to re-occur as long as you are on the highway. So we took it to the shop today since they plan on using it for an upcoming trip and want it to be in good running order. They ended up changing out spark plug #3 due to a misfire. They said it had misfired 144 times, not sure if that was 144 times today or over the course of the last 2 years. They said they found nothing else wrong. Now, I never put the two issues together (the surging, then the following torque converter / gear drop). I understand the bad plug could cause the surging on the highway, but could that surging cause something in the computer to make the trans drop down? I really hope thats all it is, as they don't really want to put any more money in the car.