Transmission Problem?

I have seen a couple of threads about this problem I am encountering, but have found no apparent answer. I have a '94 V6 3.2L mustang, and I believe it is having transmission problems. I will be driving along, when all of the sudden the RPMs will jump up as if I am in neutral. Then the car will suddenly remember it is supposed to be in gear and slam hard into gear, as if I were neutral dropping. One mechanic told me I might have a broken spring in my piston? That makes no sense to me, but 3 other places i called say it sounds like a transmission problem. It usually does it between 30-40mph, and rarely at higher speeds. It usually does it also after a long trip, usually highway miles. I am not a huge car buff, but I think I understand that AODE is the top gear, is the problem something to do with that?? I hope someone has experienced this before and has a clear cut answer, but any advice or suggestions will be just as helpful. Thankyou,
Charlie