Thanks for the replies guys. I haven't tried different plugs. The only hobby shop within 100 miles carries only 1 brand of plugs.
I'm definitely not running the engine lean. If anything, I run my engines too rich. I don't think that could cause this problem...
I'll throw a third plug in the engine and post on here if it dies within 5 flights or not.
It's on my trainer, which doesn't get as much action as some of my other ones: only when my wife wants to learn some more about flying. Could long periods of inattention cause this?
Cheers!
G