I'd try leaving a glow driver connected to it for a few minutes just to see if it changes anything. The best way I've found to do this is to connect a remote glow ignitor to the plug, you know, the kind that has a wire that runs to the plug and then a fitting of some kind that you mount on the plane, to which you then connect your glow driver. If you just leave a regular glow driver connected to a YS and leave it running for very long, the head will sometimes get so hot that it will damge or at least distort the glow driver's tip, so using the remote stops that.

This will just give you a little idea of which direction to look. If it "solves" the problem, then you might suspect a rich condition, if it doesn't then it may be fuel delivery related.

I don't think it has anything to do with the engine overheating or of a change in any setting or clearance or anything like that, since it starts agian instantly with no time for the engine or parts to cool. Sounds like a tough one to track down. Good Luck!

David