I have a 2001 A6 4.2 quatro that seems to have slowed down significantly. I took it to a Euro mech to see check it out after i changed my sparks and battery, pumped it chock full of injector cleaner, full synth oil change. Its sporadic whether there's a decrease in HP or not, but the one time it does always seem to handle best is when its cold.

 

It would be worth it to me to put a high performance car like this on the computer and get it scanned in real time to see if all the sensors are up to snuff. A tired O2 sensor or MAF can really sap power. Most cars after 4-5 years are 20% down on power due to senor degradation. I alwys suggest that to tune up a modern car has little to do with plugs and everything to do with the powertrain management system.

 

I neglected to mention it only hesitates when warm. Im going in for a full tune up and diagnostic some time soon but the diagnostics the mech did discovered nothing. The 20% slow down for being older is very understandable but its a lot more then 20, and very sporadic, sometimes at 100ish. And thank you for your help.