Bad Petcock, New Air Filter, I dunno.......

So I bought an 06 V-Star Custom 1100 last week. Looks beautiful, runs great, well except......

The guy I bought it off of told me that when he rode in situations where the air was "heavy" (his term for the air down by the Ohio River) the bike would sputter and maybe even stall out. I thought he was nuts, especially when he told that flipping the petcock over to reserve seemed to fix it. But today, when I was riding home on River Road, it actually happened. The air did seem a little damp, and all of a sudden the bike starts to sputter, then eventually it stalled out. I flipped it over to reserve, even though I had just filled up less than 50 miles before. Sure enough, the bike starts right up and away I go. About a mile down the road I flip the petcock back to on, and within a mile it's back to sputter so...

WTF? Anyone got an explanation for this? Moist air sounds a little like BS, especially since I don't think flipping the tank to reserve acts as a dehumidifier. Do I have a bad petcock?

Any help is greatly appreciated.