Tee up with good (sympathetic) stealer/indy/trans shop to have the car next time it fails and tilt tray it to them (or drive at low speed - your choice). Let them find codes etc if there are any. Could be bad/corroded electrical connection. Does it happen in the wet or after wet days? Could be a rubber boot off a plug - letting in water. As it is not constant I am not thinking gearbox internals.
Good - Fast - Cheap: Pick any two.
E23 735i; E32 735iL (X 2 - 1 Alpine White and 1 Glacier Blue); E34 535i; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E30 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i