2005 Ford Focus Diagnostic Code P2303

I have a Ford Focus 1.6 5 Door Hatchback and have just got back from holiday racking up 1200 miles. The engine management light (amber) came on and the car lost power sporadically - sometimes on the motorway I would drop from 80 mph to 50 mph despite have my foot to the floor. In other instances the car would judder when leaving a junction. This only happened sporadically mind and a lot of the time the car would run smoothly despite the light remaining on. Actually a few times the light would go off only to go on again later. I have done a diagnostic check and have got a P2303. So am I right in saying that it could be one of 3 things causing this - 1 or more plugs defective, wiring to the coil loose/defective, coil pack needs replacing? Because this was only sporadic is it more likely to be loose wiring? I also got a P0156 reading which I'm not too bothered about - this has happened on and off for a while - just thought I'd mention it.