I had this problem on my car....the engine was misfiring and spark was breaking down at high RPM (4000 or above). Had the workshop check it out and found that 2 of my coil packs were on the way out. I bought 2 second hand replacements from the for sale section on SAU replaced the old ones and now everything is sweet. In the meantime while running with the faulty coils the workshop regapped the corresponding spark plugs closer in order to try and reduce the misfiring. As more load is put on the coils, the spark breaks down more and arcs out causing the misfire. Check your plugs as well because there could be a lot of deposit on them which may be fouling them as well.