99 Transmission problem

My trans is acting weird. 1st it was dumping fluid when on the highway, the o/d light was flashing, then the trans was shifting bad. I found out that the reason for this was because the fluid was bad. I changed that 1-2 servo o-ring (helped a little), then got the trans flushed at the dealer. The trans shifted perfect for about a day...now, it's doing something weird. Sometimes the trans would seem normal, then other times it would shift hard from 1-2, then other times it would slip from 1-2. Also sometimes when slowing down, still in 2nd gear, then accelerating, there is a late downshift and would engage late and bang in. Any suggestions?
I was reading another post and said something about if there is a bump, then it is a band problem. I do hear a bump occationally about 1-2 seconds after a stop, sounds like it's coming from the rear though. I already checked everything out in the rear and couldn't figure out what that noise is, maybe it's the trans?
Also, if it comes to rebuilding the trans, can it be rebuilt on the van, partially or fully?