'97 Park Ave shifting problem

When the car is cold, it runs and shifts just fine. However, after the car warms up (approximately 15 minutes) it starts to chatter with every shift. At first this problem was only observed after warming when it shifted into overdrive, now it has a chatter (rough shift) with almost every gear. A while ago a trans guy suggested adding Lube Gard, it worked for a while and now its back and seems to be getting worse.

Any ideas are greatly appreciated.
K

my suggestion is to first eliminate the possibility of engine performance to affect the shifting. Engine problems causing the increased use of the throttle will directly affect the trans. Low fuel pressure, restricted exhaust, incorrect timing can cause this. The trans has always been affected by throttle position. Ray

I'll have the engine check, Thanks Ray