I agree with Freebird, wouldn't be the first coil I've heard off that breaks down with heat. I'd swap it for a trial or test it with a multimeter if you can. Have you pulled the plug for a look after it's been running? If you look up the net you should find a good spark plug colour chart that may shed some light on whats going on in there. Lastly, I've had all kinds of heartache before and fixed it with a new plug. At worst you have a spare plug for next time.
