Bouncing rear delrailleur

[Pauluk]

I've just cleaned a very dirty chain on a road bike and measured its stretch at 0.75. The gears change quite smoothly but I've noticed that when the chain is on any of the 3 largest cogs at the rear the derailleur jockey arm bounces up and down every few revs of the cranks. The jockey wheels seem to be ok on inspection and this doesn't happen on any of my other bikes.

Has anyone seen this before or know what the problem may be.

Thanks as always

 

[fossyant]

Sticky chain. Check one of the joints aren't stiff.

The other think is if once this is clear is the b screw that is on the rear mech. It's near the back and pushes onto the frame drop out, you may need to screw it in a little to push the chain off the sprockets.