I think the valve bodies are known to clog up.
I would guess a rebuild is in store. Happend to my buddies 94' 740, had to have the valve body rebuilt. It wouldnt go into gear until you revved it a little.
I think after it has initiall engaged(after you put it in drive or revrse) you are fine until you take it ut of gear and put it back in.
We all get the urge...Saturday early morning...nothing to do till noon...
The aural sensations of 1st gear WOT
**CGM **(Certified Garage Mechanic)