Hi folks. Bought a 2006 S4 automatic with 63,000 miles five days ago (clean carfax, 2 owners) and have noticed when cruising steadily over flat land in high gears at 1,500-1,800 rpms or 40-50 mph for the first five minutes or so of a drive before the engine is fully warmed up the tach bounces consistently 80-120 rpms. Because it's the V8, it's quite noticeable and, considering I just bought the car, somewhat unnerving. I'm in Connecticut, so the weather is nice and mild albeit a bit humid right now; i.e., harsh temps are not the issue. Video of the tach in action follows. Any help would be much appreciated. https://vimeo.com/43782842

 

I am not sure about your Audi, but many cars with AC and automatic transmission, release the torque converter clutch whenever the AC compressor cycles on & off. This is done to mask the effect of the change in engine load as the compressor cycles. Try driving with the AC turned off an see if the RPM change still takes place.