These engines can be prone to interference problems if the non genuine plugs are fitted, also alot of people don't seem to tighten the plugs down enough, this is important that they are tight enough so the heat can be taken away from the plug in to the head, if they are loose they will over heat and quickly burn out. Some people say that some factor coils are rubbish and some are ok. Me i personally only fit genuine one to my customers cars, but then i'm not paying, but since the green top ones came out i have had none back faulty. I would re check what you have fitted check for cracks in the plugs that may cause arcing out.