My A6 2.6 has recently been playing up, when you stop at traffic lights with the engine at norm temp, the engine will sometimes rev up by itself to 1500 rpm and then cut out or it will run rough showing 500rpm ish It seems to be worse if you sit with your foot on the brake, when its cold its ok. its manual trans, Also if you turn off the traction control at about 70 then plant the throttle it goes alot better than with the traction on.

 

Several things come to mind. Need more information, mainly about MX. How long (many miles) has it been since your plugs where changed? Are you using the high octane gas that is recommended, or are the high gas prices killing you (like everyone else), so you save $4 on every fill up? Air and fuel filter???? Injectors need cleaning; some people recommend injector cleaner (especially in cars with more than 50K miles) injector cleaner with every oil change. Oxygen sensor; PCV??? Many items, but most just routine MX.

Just my two cents. If you could provide us with a little bit more background info on the car we can be more helpful in your diagnosis.

Brian