Our 2002 Buick LeSabre shifts very roughly at times. Seems most noticeable at about the 50 MPH shift point.

Drop and clean the pan, change the filter, and refill with Mobil 1 Multi-Vehicle Synthetic ATF. If that doesn't solve the problem, the clutch packs are likely wearing out, due to lack of preventive maintenance.

Here's how to easily change the fluid yourself:

https://www.askmehelpdesk.com/cars-t...ml#post1873132