AOD with Shift vibration * what is it?

[sandy95gt]

Our Mustang is a 1995 GT, for the past two years we noticed a slight vibration and noise right before it shifts up into the next gear.

it got worse, and we found that if you know when to let off the gas, you can avoid it.

I had an engineer tell me that it needed a fluid change. so at 45,000 miles we had that done by the dealer. The problem went away, and it shifted nice and clean.

Then after about two - three weeks the problem is back, and at about the same level it was before.

Anybody have any idea what it is going to need? The car is in storage until it stops snowing. April I hope! I want to get it right before I put it back on the road.

Thanks,