I have an A6 Avant TDI - some time ago it started to miss when accelerating and when touching 3k revs. The problem is worse up hill. There is just no power if the accelerator is pressed aggressively. Driving at 2K revs and 70/80mph on motorway in 6th gear no problem but then revs at 2K. Seems the missing is problem is getting worse. Could this be expensive ?? I've had the car from new 2006 model 72K miles.