I have the same problem that another subscriber had with his 2000 buick. I have a 1997 buick park avenue, turbo charged. I too have a problem with intermittent hard shifting “after” warm up. I purchased my buick with only 30,000 miles. On one occasion, shortly after purchase, I experienced excessice slaming into each gear for approx. 10 miles, stop and go. Then suddenly it cleared up to some degree. After that for the remaining 60,000 miles the hard shifting is like going across railroad tracts at 30 mph. The dealership says I need a new tranny. A retired tranny mechanic told me that it wasn’t the tranny but the converter. So I still drive it with the hard shift not really knowing what the problem really is but wanting very badly to know.
Another case of a transmission running with the wrong fluid pressure. Could be the transmission pump or the pressure control system.
If you want to know so bad then have the computer scanned for codes and post back with them. We can go from there.
Could it be the wrong fluid?