lets see i had this same problem, so i took it down to the tranny shop, and they said it needed to be rebuilt. so i have it rebuilt for 900. they didnt say anything specifically that was wrong with it.
Usually when that happens it is because the seals inside the trans are hard and brittle from old age . They don't seal well when cold so when they warm up they expand and begin to seal. They sell several trans additives to help this problem and they do work for a while but it is just delaying the time untill you have to rebuild it.
A forum community dedicated to Ford Mustang owners and enthusiasts. Come join the discussion about performance, builds, modifications, reviews, engine swaps, classifieds, troubleshooting, maintenance, and more!