Talk about timing!

Oddly enough, this is the same problem I've been having with a 600 derailleur. This afternoon I put 'er up on the stand and took a close look at the idler pulley wheel and watched closely as I ran through the gears. I noticed that the chain acted "jumpy" at one spot on the idler pulley wheel: sure enough, one of the teeth was really worn down. I ran down to the LBS; he had a pulley wheel that he just gave me (great guys!) Ten minutes later I had the new one on, the old one in a recycle box, and the chain was running smoothly. I went out and rode a quick eighteen miles with nary an errant noise from the rear!