I have a 2000 pontiac bonneville ssei and sometimes whenever I am driving it it shifts hard but most of the time it shifts smoothly. Is there something wrong with my transmission that it shifts hard sometimes? The transmission is an automatic.

Most likely a bad trans mount. Check them over. If the trans was bad I would expect it to happen all the time.

If it not the mounts look into converting to Dextron VI and installing a shift kit. 11 months ago I was convinced I needed to rebuild the trans, did the Dextron VI and the shift kit- and the trans is running better every day- 30k miles later.

If it engages into D and R hard to it could be a vacuum problem. I had hard engagement into D and R also 1-2 shifts were pretty hard and 2-3 shifts a little harder than normal. after a brief look under hood I found vacuum line had been totally knocked off trans. Stuck it back on fine ever since.

The 2000 Bonneville does not have a vacuum modulator. You may have a bad pressure control solenoid. Scan for codes and report back.

Thanks for the info I'l have to remember that

Remember, not all transmission codes will set a SES light.

Correct, especially P1811, which is a pressure control solenoid related code.