89 NewYorker ... if not the tranny, then what can it be??

I am at loss on what the problem can be. :banghead:

While driving, my speedometer suddenly starts to go crazy and then maxes out. As soon as it does, my car starts to buck (like when a fuel filter is clogged and the car wont go) and I need to take my foot off the pedal and hope it calms down before I step on it again.

It has happened on the road and on the highway.

I have taken it in to the tranny guys who replaced the fluid and filter. They say the tranny looks very good. Nothing that sticks out as being in need of a fix.

Anyone have this problem or knew of someone who did and what the fix is ??

Thanks

John