Not to be too obvious or off topic, but you might try checking your rear derailleur hanger alignment. I'm amazed at how much a small angle effects shifting on the rear. And, equally amazed that even though I baby all of my bikes (when I'm not riding them) I still manage to bend hangers enough that I have to get out the tool to straighten. When I find one bike that is out of alignment I usually just throw them all on the stand to check.