Have you pulled the plugs to inspect for oil on the ground strap, bottom of the shell and on the insulator. Ideally, the plugs should look something like cigar ash, with perhaps a little brown tint. They should be perfectly dry and not oily. If the plugs are oily, then look at the spark plug seats to see if some fuel and oil could be blown past the seats in the heads. Any dinging or damage to the seats will cause problems. Are they the proper spark plugs for the application?