first check for chain kinks. just freewheel the pedals backwards and watch the rear derailleur- see if the cage hiccups from a frozen link. easy fix.
as everyone said, check the chain- past a certain point and it will damage the cassette. i get several chains per cassette. performance has a cheap measuring tool, or you can measure it with a ruler too- google for chain length per x links. i don't remember. the tool is easier.
another, easy fix is the possibility that your sis or derailleur pull is off, possibly from cable stretch/slippage. also make sure your cable isn't frayed. if the rear d is set up properly at limits, check the sis setting. shimano has the methodology for set-up for rear d and sis on their web site. pretty simple and quick. sometimes the sis can work without load then slip with load. twist the adjuster to see if it gets better or worse and repeat.
if all else fails, shuttle.