51 plate Megane 16v engine lumpy?

Driving home my engine decided to turn very lumpy. Thought it might be stalling but limped home. On the way the engine management light flickered on and then off. Restarted the car and the same fault. Seems to be only firing on 2 cylinders. Checked and cleaned the spark plugs to no avail, still not ticking over properly. Where's the next most likely fault going to be and how do I check it out ?

Rich
December 2006