finally took it to mechanic and he said only use premium fuel. I had been using regular gas for a while so I changed and no further problems found! It must have been something to do with the timing being retarded automatically by the ECU and causing too much current to flow to the plugs. Unfortunately, it now has a small leak in the head gasket which shows up with a cooling water test but otherwise no problem, runs sweet. Also well past its date for a timing belt replacement. What should I do now?