Look for something delivering too much fuel or not causing an adequate burn (like timing). If it's bad, like hardly driveable, it's probably timing or very poor ignition. If it's slight (annoying) then look at something that would cause it to run rich, i.e. bad MAF, coolant temp sensor, stuck injector, etc.

If it's a crappy idle I'd look at EGR and the vacuum issues that everyone has...