The not running at idle is exactly what my 740t did when I was a youngun and not driving. It happened one morning (trying to catch a train out of town) and it turned out that the timing belt partially failed, then completely failed while driving down the road at 60.
It seemed like some "teeth" wore down and slipped a few notches before it broke.
Can you check the timing with a timing light? I hear they're fairly easy to "make" yourself. Or about 30 dollars.

If you put up the hood all the way, can you see around it to watch the dizzy crank? Know what I mean-- all the way vertical - there are some little stops that rotate and allow the hood to go perpendicular to the ground for removing the engine or working on things in the back of the engine.

Good luck, Post back,
Will
--
1990 740 Turbo, on its way to stock specs, maybe beyond