I've been riding in the rain a lot lately (though I don't know if that's related), and my rear gears keep automatically changing on me. I first noticed it while pounding up a hill, and then my bike decided to shift into a harder gear, completely destroying my rhythm.
At first, it was just like that, where it would happen when I put a lot of torque to the pedals. Usually while on the second cog, too, slipping down to the third where there is quite a large difference in teeth count.
I tried tightening the nut on the shifter. I have downtube friction shifters, by the way. But it didn't work exactly (so lately I've been tightening and loosening it while on the go, just to pretend like it'll be fixed). Now it just seems that the shifter is having problems trying to find the gears. It seems totally out of whack. Like I'll have to move the shifter farther than I remember to get to a certain gear, or that the distance the shifter moves in between gears is not even and feels sporadic. On top of that, I'm still having problems getting the chain to stay on 2nd, or now that I think about it, I had problems getting onto the smallest cog, too.
I'll go back again and look at the derailleur settings, while I wait to see what you guys have to say.