Car runs and drives great until you shift into overdrive, then car acts like it's shaking or stumbling real bad. Downshift into passing gear and no issues. What could this possibly be?

Hi, this is Bo, an auto expert on RepairPal's Car Genius team! If your Buick stumbles and shakes only in overdrive, it could be a torque converter clutch (TCC) issue. The TCC is designed to lock up in overdrive for better fuel efficiency, but if it's malfunctioning, it can cause that stumbling or shuddering feeling. It’s like trying to ride a bike with a bad chain—it’s smooth until you hit that gear. Another possible cause could be misfiring under load or a transmission issue, but I’d start by checking that torque converter lockup. Avoiding overdrive is a temporary fix, but the issue won’t disappear on its own. If you need any further advice or want help finding a trusted RepairPal shop in your area, please feel free to call us at 877-913-1554.