Thank you - your English is better than mine ... and I only know English
100C would be 212F and I can easily see the stator exceeding that temperature.
"Thermal cycling" of the stator from a high-side temperature that is too high seems a very likely explaination.
I wonder - Is there a better grade of insulation available?
VTBob: I see your point on not wanting a de-tuned stator as the solution ... you Vermont-riders likely need all 400W from time to time. The good news is that when you need the 400W the outside temp. is so low the stator may not care?
My hope for insulating the exhaust header was not that it was a real CURE, but only that it would help things a bit ... esp. during low speed operation in hot environment....