If left idling for a couple of mins (on the drive to warm up/ de mist or in heavy traffic) my gf's 51 plate 1.6 Leon starts to fire on 3 cylinders. The engine fault light on the dash flashes then after about 10 mins goes constant . When the light turns constant the problem stops and it runs ok, after a day the light goes out and its seems ok. We've changed the EGR valve, plugs, ignition leads and transformer but still has this issue. Any ideas?