2001 Bonneville with 160,000 miles on it. Trans was changed once and the dealer put in a rebuild. i think that was around 65,000 miles. It shifts fine when cold, when it warms up, it'* starting to shift hard. When it cools down it shifts smoothly. It has never had it'* trans fluid or filter changed. Is that where I should start or is there something else I should be looking at?? Other than the trans, this has been a great car except the rust by the gas fil.

Jim

Using a shop scanner, Scan for PCM (OBD2) codes, the hard shifts may = the P1811 solenoids, long shift adapt problem.
Often installing a Transgo 4t65E shift kit (springs & spacers) will stop the hard shifts.
In the meantime since varnish can build up in valve bodies and solenoids, I would add a can of Seafoam, Trans Tune and see if that makes it shift smoother when hot.
Probably a good idea to drop the pan install the shift kit and use Dexron VI (6) fluid in there, with of course a new filter.

Verify that it isn't the front motor mount issue. It would simulate a hard shift when the mount "breaks down" internally or separates.

Thank you for the input, I will have it scanned at Advance auto today and then I will check the motor mount. I need to find a way to siphon out some fluid in order to put the seafoam in. Then find a shift kit and do a filter change. The trans calls for dexron 3 but your suggesting dexron 6. Are they compatible as I only planned on changing the filter not draining everything else.
Jim

Yes the two transmission fluids are compatible.
Dexron VI is the latest standard of GM fluid and will offer better shifting and cleaning properties.

Thank you for your response soft ride. Is there a good way to check the front motor mount? The transmission is not showing any codes.
Jim

Lift the hood here is what it looks like.

Here inside mine was torn, and ya it clunked with some shifts.

The movement test is:
In drive with wheels blocked, rear brake and pedal brake on, give it some gas and with someone standing off to the side, visually inspect the interior movement.
The bushing will compress a bit but no clunks should be heard and no sideways twisting motion should be observed.

Thank you, I will check this over the weekend.
Jim

That mount looks torn.

Thank you all so much, I replaced the front motor/trans mount and it solved my shifting problem. The front mount was torn. All the local auto parts stores had it at the same price so I ordered it on line from Advance Auto and saved 15% and had to wait half an hour before I picked it up! Thanks again.
Jim