I ahve a 2006 Envoy Denali V8. When it is driven for more than 5 miles or when it gets up to interstate speeds it seems to shift hard from 1st to 2nd. If I just drive it around town then it has no problem. I have been to the transmission shop and they said it wasnt the transmission so Im confused??Can anyone give me some advice? I have been told about a Transmission Control System Sensor?? Could this have abything to do with the Throttle Position Sensor? Thanks for any help.