91 Mustang Gt Tranny Prob

Just wondering if someone on here might know what happened to a 91 Mustang GT I'm most likely going to buy, when I first heard about the car the guy who had it said the only problem was the tranny slips a little bit, he was letting someone test drive it before I got there and while they were test driving it they pulled over to get gas and here's the problem-When they get back in to the car to start it, the car starts pulling forward while cranking, kinda how a standard would do without the clutch pressed, he also thinks it broke the timming chain, but what could explain the pulling while cranking? I have never heard of an auto tranny doing something like that, anyways thanks for any help you may have cause we're stumped.