Coilpack Fault
 

Often the coilpacks on these engines go wrong, typically after a heavy handed mechanic changes the spark plugs and knocks them around a bit.

The car might start fine and suddenley the engine check light will appear and the car will appear to be running lumpy.

Do not drive too far like this as it can damage your engine. The A8 D3 has cylinder detection for a misfire and by using VAGCOM or other diagnostic devices it will tell you which one.

Coilpacks sit on top of each spark plug.

Also, there is an ECU firmware update to fix random misfire errors. Might be worth having this done before you change coil packs (It usually shows up in pairs apparently).