I have the same problem with my 93 740I. It has 130K on it.
It just recently started to display a delay in the reverse gear engagement and the other day it just sat in idle when the shifter was put in reverse.
I took it my mechanic and he said it needed a $6500 (Ouch) factory rebuilt tranny. I called a few transmission shops and they state that they can overhaul it for a few grand. My mechanic says I'm just looking for trouble. I picked it up at the shop to take it to the transmission shop and now the reverse is working. I guess it's a ticking bomb. It seems that this is a common problem on the 5hp30 transmission.
Any suggestions out there?