My 97 Sierra pickup 350 V8, auto tranny runs fine at all speeds, but when you press the accelerator hard enough to make it shift down, it starts to miss badly. Acts like it's only running on 4 cylinders, chugs and looses power totally. If you ease up and let it shift back up again, smooths out and runs like a top. You can be running along at 70, come to a big hill and press hard enough to make it drop out of O/D and it misses. You can accelerate normally as long as the tranny doesn't shift down.

Does anyone have ideas or suggestions?

does it do it if you downshift manually?

Darn good question, I'll find out and post. Thanks for the response.

It only seems to do it when you press the pedal hard enough to make it shift down to the next gear. Runs ok if I shift down manually.