Does the shifting occur whey you stand on pedals (I.e. frame twisting?), or after you have spent some time on a given gear (i.e. derailleur slowly shifting), or after you have upshifted or... ?


Apart from above suggestions, here are a few :

- Check if the spring inside the derailleur is jammed with crap or severely rusted. The derailleur should be able to move freely under spring tension.
If necessary, flush the derailleur spring clear with WD-40 (with the straw) and re-lube.

- Check that cable isn't kinked anywhere along its length. A kincked cable won't move as well and would provide very late shifting.

- Check indexing close to the largest cogs. Since the upper pulley has some built-in lateral play, try to backpedal and see if it remains in the middle of its zone.

- Check the tension screw where the cable enters the derailleur. Believe it or not, I once had played so much with it that it was unscrewed out of the derailleur. It didn't look bad because cable tension kept it in place, but it also meant very unreliable indexing. Try to wiggle it; if you can move it sideways, you need to screw it in place.