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.

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Good - Fast - Cheap: Pick any two.
Current:
E23 735i; E30 318i; E32 735iL; E38 735iL; R50 Cooper; R55 Cooper Clubman.
Previous:
E21 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i