I might be on the wrong track, but what fuel are you running? If over 5% you may be detonating and destroying the plugs. That would also account for the overheating.
I've had to open up the combustion chamber, a trick described in the notes now at Fayeteville:
http://www.fayettevillercclub.com/id59.html
If not had a boo at these they cover some good points on the 40.