transmission shifts hard on 1997 Buick Park Avenue

Tranny shifts fine all gears cold but shifts as though going over car counter cables when hot. bang, bang, bang

Asked by for the 1997 Buick Park Avenue
First thing is to make sure the trans fluid is at the full mark
1 more answer
I'm told this is probably a solenoid of some kind - if outside the trans casing, reasonable to replace but if inside the transmission, no shop will open and replace, they want to put in a rebuilt transmissioin for about $3500-$4000. With 185000 miles on mine (and i love it!) guess i'll wait for the next Lucerne and see.