How is the chain? Pretty clean and adjusted properly?
Also the shift cable? It might not have any kinks in it, but is there any slack down near the derailleur or up in the grip-shift unit?
Try lubing & cleaning the chain, and maybe put a drop of Tri-flow or other light oil on all eight of your derailleur pivot points. In fact, shift it to the largest cog, lube those pivot points, then to the smallest cog and lube them all again. Then wipe off the excess and see how she does.