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Acceleration problems following transmission rebuild

I recently had the transmission rebuilt on my 2004 Buick Rendezvous. After 150,000 mile and noticable slipping, it was time. When I got it out of the shop, I noticed that acceration was sluggish, When you would stomp on it, the engine would start to “boggdown” while the RPMs would hit 5000. Took it back into the transmission shope where they checked it out and said transmission is operating fine, but they show an o2 sensor issue. They suggested that it might be this snesor or that it may need a tune up or perhaps just new plugs.
I took it into my mechanic and they checked out everything, plugs, fuel filter,and repleaced sensor. Afterwards, they said that they get no misfire codes whatsoever, and say the engine runs great. Unfortunately, the acceleration problem did not go away I am stumped. The mechanic says its the transmission; the transmission guys says it’s the engine…all I know is I cant afford to keep throwing cash at this vehicle, but really need it to last for another year or so.Help!!!

The engine isn’t “boggdown” when revving to 5,000 RPM while not increasing in speed, it is flaring. In normal operation how does the transmission operate? When shifted to 1st gear and driven up to 20 mph will it accelerate without the RPMs flaring.

But first, can you drive a manual transmission? Are you familiar with a slipping clutch? Flaring is a slipping clutch. There are several in your transmission that are internally controlled.