It could also be your derailleur hanger. In general, if your shifting works in the first two-three gears but gets whacky elsewhere, then you adjust that whackiness out only to find it moves elsewhere on the cluster, the issue is often a misaligned (or loose, or both loose and misaligned if it's bolted on) derailleur hanger. Your LBS should have a tool for that. Make sure your rear wheel is relatively true before you bring it in as the tool indicates against the rim It's possible to realign an aluminum hanger, but for best results, just get a new one.