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I had this *identical* problem with my A6 2.8, and my neighbor with his A4 2.8. In both instances, it was a faulty coolant temp sensor. The problem has not recurred.
Yes, I too have had this identical problem with my 2001 Audi S4. I just replaced the coolant temp sensor, but that didn't work. I'm now going to look into the crank shaft and cam sensor and see if it's one of those.