206 1.4 running on 3 cylenders?

i have a 206,1.4 petrol which has started running on 3 cylenders.The fault has corrected it's self a couple of times but started again when i went round a large round about. it's been on diagnostic equipment including a peugeot one and no one knows what the problem is. the first diagnostic said coil fault but that has been ruled out.i've checked for obvious breaks in cables etc..