what rugen said. I had a problem like that which turned out to be a mis-stamped (surley) cog, so the wheel was true, the crank was centered, but the chain was still loose enough to pop off, or so tight the cranks would barely turn. If it still persists, take the rear wheel and spin it, and watch the cog... if it hops up and down, that's bad news. replace cog before riding on.