I have a 2004 LeSabre with 128K miles. At 100K I Had a dealer change the transmission fluid by reverse flush. I went on a cross country trip and the transmission would slam into an upshift ocassionally. I have since taken the car to a local mechanic and had the transmission filter and fluid changed. This did not correct the problem. Any ideas are welcome.
First, be sure to find out exactly what type of fluid they used during the flush process. If they put the wrong fluid type in, it can cause harsh shifting.
Also, your owners manual should have service intervals for trans servicing. What are those intervals? Have you had this done before? What type of ATF does the manual call for?
Waiting until a trans has alot of miles before servicing it can actually lead to having problems.
Since it was a GM car dealership that did the service I have to assume they used their DEXRON fluid. The owners manual said to change at 100K interval. I had it changed each 50K-60K. This is the first time I had it reversed. Every other time I had the pan dropped, replaced the filter and fluid. The Owners Manual calls for Dexron III. I have lost any and all confidence in this GM Dealer.