I would check for possible water ingress into the electronics box area (as I call it) under the bonnet. It's up on the right-hand passenger's side next to the firewall and has an easily-removable cover. This may be different on a 2.8 to the 4.2 though that I have. On my car, the ABS box was I think the lowest one, and then my gearbox TCU sits above it. Bearing in mind the recent rainy weather, I suspect that this area might be full of water. It's certainly worth checking.
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2010 B8 S5 Sportback quattro 3.0 Supercharged in Sprint Blue (current girlfriend)
2000 D2 A8L quattro 4.2 in Audi silver (my ex-girlfriend)
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