Hi James, I dont know whether this will help you or not, I had a passat that would run perfect for a week no problems, then it would start to hold back, and idle quality would be erratic, no codes stored, and everything, and I mean everything checked out OK. fault only lasted for a short time, too short a time to get a bead on it. looked at the wiring diagram, and engine control relay appeared to be favourite, my thinking was that the reason no codes present, ECU thought ignition was being switched off, took the cover off the relay, and contact was blue with a weak spring, renewed with a modified relay, no more problems.