Mine was doing something simular, but I was dropping gears. Like if I was in Manual mode I would be driving in 6, the engine would get loud and then drop to 5th or even 4th.
If I was in drive or sport drive modes it would get loud, shudder and then buck bad.
Took it too the dealer and they found a hole in the top of the CVT where fluid would be leaked out. It would only leak when running as the presure was there to force the fluid out. So they dropped the transmission and put in a new one for me. No problem since. That was at 17K I am now at 30K. No more problems since. My car is a late '02 build (but a '03 Model).
Winslow
