Buick Repair Questions? Ask a Mechanic for Answers ASAP
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Since you had the fluid flushed, the fluid level should be okay. The problem you are experiencing is due to a failing forward sprag clutch. This clutch is purely mechanical and is supposed to lock up when accelerating and unlock when coasting. The problem is that the dlutch is not relocking when accelerating after stopping or slowing down.
The fix for this is to replace this clutch. The transmission does need to be disassembled to replace this clutch.