2003 Buick Rendezvous transmission

check fluid level and scan for codes it may be a valve body problem or clutch problem.

To me it sounds like the catalytic converter might be bad and starting to clog up, when this happens the engine vacuum is lower and it makes the computer think you are going up a hill. Here is a guide to help test the cat and the cat location in the images below.

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

Also, let's check for codes

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

Check out the images (below). Please let us know what happens.