i had the same problem, ruled out the possiblilty of leads after replacing them then got really pissed at the whole car, found out after that the guy that fitted the leads damaged one of them as he was fitting it (hard to fit) so had to replace a single lead, now is working fine, when it is dark, leave the car running and stare into the engine, where the plug leads are based, if you see a spark then you know the problem is with one of the leads, thats how i found the lead was damaged after replacing it.