I have a 98 Park Ave Ultra with approximately 155000 miles on it and having a problem with the transmission slipping / grabbing from a stop or very low speed only when hot. It feels like the transmission neglected to downshift to 1st gear and then once realizing it needs to accelerate then downshifts after the car is moving creating a thunk after which the car accelerates normally and shifts normally. The transmission was replaced with a used (40000 mile) transmission about 6 months and I didn't notice the issue then, though I didn't drive it much either. The filter was replaced and the transmission filled with Advance Auto parts brand Dexron ATF when the transmission was swapped out. Anybody have any non-invasive things to check?

It sounds like you have a line pressure issue that may be pressure control (PCS) related. The slipping from stop is called launch shudder. Get it scanned with a trans capable scanner and see if a P1811 (max adapt, long shift) DTC is set. Does not give a SES/CEL light.

Also, our Buick Tips & Maintenance forum has a thread covering the P1811 DTC or harsh upshifts.

Your reply is very interesting. I have a code reader that will read extended codes but I'm not sure if its trans capable. When I checked codes today I did not have any!! Does the P1811 code auto-reset once the condition clears? The seafoam and shift kit solution would be a lot cheaper than either the PCS replacement or a rebuild. I guess if I can install the transmission I could certainly install a shift kit.