I have a 98 A4 2.8 Quattro. This problem started a couple months ago. When I start the car everything is fine, runs well for a few miles until it warms up. At that point things start to go downhill. At idle it will periodically have a rough idle, if I put the car in gear it will sputter. It will even do this at low rpms in second and third. I notice that it does it more on low throttle, when I give it more gas it usually "catches" and revs freely. My gas mileage has also been suffering a bit too.

Could this be bad O2 sensors? I just changed my sparks plugs about six months ago.

 

spark plugs, wires, or ignition coil. you have a misfire

 

The Check Engine light is also on.

 

did you gap your plugs? Also (you probably checked already but), are all your vacuum lines connected... might you have disturbed them while installing the plugs?

 

Very possible the O2 sensors are causing issues. Thru don't come online til the car warms up some, which is when your problems start. They'd also explain the poor mileage. Scan the computer and post any codes you have. Those will help in the diagnosis.