Well I had a similar problem today at the races.
My 21-4C was running allright in the first qualifying heat, everything was fine.
Then the second qualifying heat, it ran great "on the bench" while tuning, then in the warm up it ran great, and then four laps in it just stalled.
Third qualifying heat, replaced the plug, made sure to get it tuned properly, spent 10 minutes running it warm and all, then sure enough it was running great in the warm up, the first four laps it was awesome, and then it stalled and my mechanic couldn't barely get it running again.

In the seconds before it stalled, on both occasions, it seemed to all of a sudden go very rich, like it was very soft on power, just the kind of symptoms you get when an engine is really rich, soft in bottom, very "progressive" sort of throttle response.

My friend suggested that it could be a bearing issue?
I'm stumped for suggestions.
When cold, if I turn over the flywheel, it seems to have decent enough compression.