Just a shot in the dark here. I had the same problem with a chev. 350. Stubburn as I am about taking it to a shop, I wore out the center deal on 3 rotors before someone mentioned that "...there was a resistance in the flow of current to the plugs". I replaced the ignition wires (I think it might have been just the coil wire (?) but I replaced them all, as they were old) THAT SOLVED THE PROBLEM!!!! Good Luck Matt