Reverse Puts S8 Into Limp

[ahd91]

Got a bit of an issue with my recently acquired 02 S8.

Whenever I put the car into reverse, I get a bit of a clunk, and all the transmission lights come on. The car then goes into some sort of limp mode in that power appears restricted, but will actually change through some forward gears, and does reverse without issue.

Restarting the car immediately rectifies the problem, and all forward gears are extremely smooth.

The problem was intermittent for a few days, but now appears to be happening every time I go into reverse. It's annoying, but I can live with it if it's no threat to the transmission as a whole.

Ideas?