Audi A8L 2004 out of service

My 2004 A8L has been at the dealer because the engine light blinks ( a tow in problem) since mid November except for one week in the middle. The engine is very rough or does not start. The first time a fuel rod was changed. Worked for one week. The dealer and factory rep are baffeled. Has any one else had this problem. I love the way the car handles and rides. I also saw an article in The New York Times refering to unknown electrical or computer problems in the A8L. I would like to hear from people with similar problems and suggestion s for solutions