Earlier today I installed the K&N Cold Air Intake kit on my 99.5 A4. The car starts no problem, stays at it's initial high idle for a second (something like 1300RPM, can't remember at the moment), but immediately drops to
Found the problem, I'm just an idiot. I re-installed the MAF backwards (one of those kind of things where the whole process flows quite smoothly the wrong way). Flipped it around and all is back to normal.