I am having similar problems with my 89 760GLE for four years now. She's been to multiple mechanics in multiple states. Everything seems to point towards the printed circuit board behind the instrument cluster, where the alternator and all electrical goes through. It is apparently very very delicate and teh solders wear out over time, vibration in the car can make it bend, move, and cut electrical power.
The summer it started, just driving like normal, all of a sudden the radio cut out, panel lights came on, car started sputtering, and it died. That summer, I had three alternators put in. Obviously that wasn't the problem, but that mechanic didn't know anything. I've had to have the car towed probably ten times since then. That last time happened a few weeks ago. It died, had it towed to the shop. Absolutely no juice, no turnover at all. However, it started fine when they got it the next morning and was running fine the whole time they had it. Everything was checked, alternator fine, battery fine (new), wiring harness had already been fixed. all things point to this circuit board. Problem is, I've had several shops in several states trying to locate this part for years and no one can get it. My car is in great shape, rebuilt transmission, engine completely taken apart a few years ago and everything replaced, no leaks, I don't want to get a new car, but I can't keep feeling unsafe when driving because there is no warning before it goes. So, you should look in this direction since it sounds pretty similar to my issues. That said, does anyone have any idea of places that stock hard to find parts? I live in NY. I'd love to fix this last issue and I need that elusive part! If anyone has any suggestions, I'd be thrilled to hear them! Thanks.