Well, it could be a bad transmission or.. computer , speed sensor.. Have it checked out before you do anything.. Good luck.
Best to take it to a transmission shop for proper diagnosis.
If you're experiencing hard shifting you may have lost overdrive and torque converter clutch lock in. It's common with these transmissions. Hard shifting is a secondary symptom as the computer has increased the transmission fluid pressure to try to compensate for the problem. At cruising speed (60-70 MPH) you may be able to hear a whine and your tach will be around 2500 to 3000 RPMs. On a normal transmission the tach should be between 1500 to 2000 RPMs at cruising speed. HTH. -j
Looking for a Used Grand Prix in your area?
CarGurus has 55 nationwide Grand Prix listings starting at $8,950.
Search Pontiac Grand Prix Questions
Pontiac Grand Prix Experts