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.
3 x E32 735iL (Alpine White, Glacier Blue & Kashmir Beige); 2 X E38 735iL (Oxford Green & Alpine White); E53 X5 4.4i (Pearl Beige); R50 Cooper; R55 Cooper Clubman (Both Pure Silver).
E38 Parts Car
E21 318i; E23 735i; E30 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i
I've just changed my wiper blades and now my reverse lights don't work. Should I change my fuel pump relay?