You could have an E4OD or 4R70W(AODE-W). Both use all-electonic controls, so normal automatic transmission diangosis may not find your problem. Get a code scanner or have someone read diagnostic codes for you. Based on my problems with an E4OD, your fluid pressure is being regulated high in an attempt by the computer to compensate for another weak component in the transmission. Sometimes, just loose or dirty electrical connectons are the problem.

Jim