Shifting

The accumulator spring could be broken or the piston could be sticking. The transmission main line pressure could be low, or maybe one of the valves in the valve body could be sticking, the shift solenoid could be bad, the TPS sensor or the vehicle speed sensor all could cause this possibly. If the fluid is full and clean, then I would think you could have an internal transmission problem that needs to be checked by a professonal transmission repair technician.

I know this may sound silly but have you checked your muffler(catalyc converter) had the same problem on a 97 camaro and that was the probem. if you go to a honest transsmission shop they will tell you for sure or just go to a muffler shop and ask them