Moisture...moisture...moisture. Intermittent faults and operation...
I had a similar issue with my '98 S8. The controller turned out to be faulty, there was moisture once it was opened up. I purchased a used one, and it was fine. I dried the old one in my airing cupboard, in a cardboard box filled with silica gel.
I re-fitted the original controller a week later, and it worked perfectly, till I sold the car almost a year later. I sold the used one on eBay for much more than I paid for it in the first instance.
