Tranny problem - overfull?

We have a 2000 Taurus AT at work and it started hesitating when you would start out in drive. It also hesitates on downshift. This was a new problem so I checked the tranny fluid. It's about a 1/2" above the hash marks. Someone obviously overfilled it. We took it to the shop we use to have it checked and it came back still over full and they claimed the tranny was probably going out. We've got it scheduled for a tranny service somewhere else. Is this a common problem if over filled? Thanks.