So this will be the first time ever that I disagree with Lynn.
While there is no doubt that the plugs he recommends may be the best, I do not use them. Those plugs are $40 each. I use the NGK BR10EIX plugs which are iridium and almost as cool, but most importantly only cost $10.
From my pocket book, good enough is good enough. While I do have high duty cycle, I don't have the peak stresses that racers incur, even in my turbo engine. I find that the iridium plugs last significantly longer and as far as I can tell they are not overheating and are producing great spark.. I tried the plugs that Lynn recommend but they only last 20 hours or so while the iridiums last in excess of 50 hours. So every 100 flight hours the cost difference is over $750! and I cant tell difference in the way my engine runs.
Steve, I'm sorry that I cant solve your problem, but I am pretty sure the problem is not the spark plugs.
Dave Leonard
Turbo Rotary RV-6