Could be ethanol. Our gas in NC can have as much as 10% ethanol in it. The ethanol will absorb moisture from the air. I have a 1967 truck that doesn't get drove much and had a very similar problem.
I have heard some people say that high test isn't as bad, I don't know. Another alternative is to find a good additive. Or aviation fuel, 100LL, which is 100 octane low lead. Aviation fuel has high quality control standards and no ethanol. However, I wouldn't mention to the guy at the airport what you are gonna use it for. I have a friend that is a pilot and he uses it in his weedeater, lawn mower etc and likes it because it has such a longer shelf life. When you put the bike away for the winter you could fill it with the 100LL for storage purposes.