Remove the rear pivot bolts, and inspect the spacer sleeve bushing for wear. Since it's a dark gray anodized surface, if any has significantly worn off, you will see bare aluminum, this can create play.
I never had shifting problems when my frame was new. I converted the cable stops on the frame by using a Dremel tool and carbide deburr bit to open the cable stops to accept full length cable from the shifter to the rear derailleur. Never had a ghosting problem, and the cable stayed much cleaner too. I used zip-ties to hold the cable to the frame and seatstay, so it wouldn't slide around under suspension motion.