I have my 92 500 SEL in the shop at Precision Motorcars where Aaron Greenberg works for this same problem. I also had problems with the oil pressure. I will call them on Sat. to find out what they think is the problem. Mine was slow to engage the reverse gears but I did not have a lock up. I posted this question before and some thought it was a reverse servo valve problem. I will let you know what they have to say about the fix.

Fred