Bicycle Mechanics - Worn Chain or Derailleur cogs?

I only have this problem when riding in the rain, but it seems to be getting worse. In the middle gears, there is a definite roughness coming from the back somewhere. It is ok in high gear and low gear, rough in the middle. It's getting worse each commute in the rain, but the problem goes away in the dry. (I always oil the chain after a rain ride). The rear derailleur is an old SunTour - shifting is ok, it's just the roughness in the middle gears while pedalling. Anybody got any ideas??

If you ride mainly in the middle cogs, they have probably worn out before the others. (I speak from years of experience with SunTour freewheels. I consistently wore out 2nd and 3rd before the other three cogs.) If you are using a wax-based chainlube, try something more water-resistant. If your chain has elongated by more than 0.5 percent, replace it immedately, but budget for a freewheel replacement, as well.

Hmm. Well, if the cogs are worn that is pretty easy to see. Just look at them. If the teeth on those middle cogs look about half the width of the unworn cogs, there ya go. How many miles on the chain? Chains only last about 3000 miles on my commuter. Pulleys don't have much in the way of teeth anyway, but if they are worn to the point of being not much more than bumps it would probably be a good idea to replace them.

Thanks, I'll get a new chain and pullies. They're old, anyways, but the freewheels are only 2 or 3 years old. I thought it might be something rain related - Everything just purrs along when it's not raining!