This problem came about when the weather turned cold. Reverse okay. Driving was a pain until tranny warmed up - it would not shift from 2nd to 3rd. Took to GM they determined bad valve body and 2nd to 3rd upshift valve(?). After talking to dealership employee, was told this is a ongoing problem that GM is well aware of. The employee stated the fix is to flush & drain tranny and check valve body operation. Luckily mine was under warranty which covered this. Also was told this problably would not have been noticed in a warmer climate.