1992 BMW 740 Transmission replaced, still slipping

Transmission problem
1992 BMW 740 V8 Wheel Drive Type unknown Automatic 172000 miles

Soon after purchasing my 1992 BMW 7401 (used) my transmission started slipping and braking itself. Then it began shifting out of gear and woud not go forward. I'd accelerate and tons of white smoke would come out of the tail pipe, but the car wouldn't go. I'd have to turn it off for about 10 mins, then restart in order to get it back into drive. Took the car back to seller and he took the car in for repair and after 6 weeks in the shop, he said the transmission was replaced with a used, but good, transmission. I drove the car for 3 days after the trans replacement and began experiencing the same exact problems. Took it back and the mechanic claimed that he forgot to reprogram the computer, which was causing the problem to reoccur. He kept the car for another 2 weeks and said he reprogrammed the trans and everything should be ok. Picked up the car, drove it home and after about 10 miles the transmission did the same exact thing again. Now I'm starting to believe that no replacement or work was done on the trans at alll. I just can't imagine that a different transmission was put in the car, and I'm still having the same problems.

Does this kind of thing sound like a normal occurence to anyone?

The seller has instructed me to bring the car back in, but I've had enough of this back and forth business, being without a car for weeks, and still ending up with the same problem. So, I'm not sure what to do at this point, but if I do take it back it, I'd like to be armed with some type of knowledge to counter what he may tell me next.

Any information/insight you can provide will be appreciated.

Thanks!