My S3 has been running sluggish for a few days now. When driving home from work today the car completely stalled on the approach to a roundabout and left me with a rock hard brake pedal. I only managed to miss cars on the roundabout by using the hand brake and both feet on the brakes. After I'd cleaned my under pants I found that the vacuum hose which runs from the brake servo to the inlet manifold had split and popped off (on a 90 degree bend by brake master cylinder). Managed to do a get me home bodge with insulation tape. I've now made a more permanent repair while I wait for a new hose from Audi. The car feels a lot more responsive and has got its punch back.