Pol441: I am having the same problem. 04 duramax, 35000 miles, used for some light towing. Most of the shifting is sporadic, but the major issue is that when full throttle acceleration the tach rpms get to 2800 and the trans will not shift from 3rd to 4th and the engine stays at that rpm until I let off the throttle, then it will shift. has anyone come up the cause of this problem?