pull the head and see what's up.
Although it sounds more like a carburation or electrical problem than a mechanical one, when you push these little engines to the limit you gotta stay ontop of things.
Heat makes air thinner so it will run richer at 90F than at 50F. Thats no problem.
But humidity in the air means the air carries more water vapor. That vapor crowds out oxygen so the carb runs leaner. (air + water + gasoline instead of air + gasoline.)
If you are riding on the edge of a too-lean mix and the air changes to very humid, jetting has to be adjusted..
Electrical.. who knows.. maybe a condenser is dying. maybe the points are floating..