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

2 answers
First thing is to make sure the trans fluid is at the full mark
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.

CarefulDriverJim