Probably unconnected, but back when I had my '96 A8, I had a similar problem. Most of the time the car would start just fine, then occasionally it would refuse. Starter turned over, plenty of spark, but just wouldn't fire up.
My garage were unable to replicate the fault, so I lived with it for a while. Then one day I couldn't get it started at all. It was low on fuel anyhow, so I wondered whether the fuel gauge was reading incorrectly. I had a gallon can available so I decided to dump it in the tank and see if anything happened.
My wife was in the driving seat and, for reasons only known to her, decided to try again as I was undoing the petrol cap. It started up immediately.
I did up the petrol cap again and we went on our way. The next time it happened, I tried undoing the cap again (only by about a turn this time) and sure enough, she started.
I took it back to the garage and explained what was happening. They said it was a clogged vent in the filler cap or tube (can't remember which - this was several years back). Never happened again after that.
I figure it was probably just haunted
