Yesterday I had the aircon compressor changed on my S3 by a mobile aircon specialist. He's done it before - this was a replacement as the first one developed a fault.
Unfortunately the battery also went flat yesterday (I had known it was on the way out for a while) and as he finished late I wasn't able to test the car.
Today I bought and fitted a new battery, which was a bit of an effort but got there in the end. But after a few mins driving the engine started cutting out, well not completely stalling but just dying under acceleration or "kangaroo-ing" along the road. When I leave the car to idle, every few seconds the revs drop right down and it almost cuts out but then immediately recovers. The ESP warning light is illuminated.
What's this most likely to be? I've checked the battery connections and I'm sure its fine - would the car behave like this if one of the connections to the battery was dodgy? And could it be anything to do with the compressor replacement. It was raining when he was doing the job and the cover was off the top of the engine so presumably some of the components could be damp.
Should be able to get a VAG-COM on it tomorrow to check for fault codes, but in the mean time I need ideas for what else to check.