Strange misfire problem

Over the past 2 months or so my idle has been getting rougher and rougher (cold or warm) there seems to be a slight "wobble " to it, more so than previously. Last week I was on my way back from a driving holiday in France (1400 miles round trip) with prolonged 130 KMH driving when I pulled up at a toll booth the car suddenly started to run on 3 cylinders and sounded like it was going to die. I revved it a few times and it appeared to be OK again and has not repeated this since. My first thought was perhaps some crap in the fuel.

Other than the rough idle and occasional surge when getting on the throttle at speed the car is running pretty well. Could this be a symptom of a coil pack? There is no misfire under load. I had the plugs changed (bosch ) a few months ago so I doubt that its them unless a faulty batch....  Any ideas?