You've changed out cassettes twice?!? Are you still on the original chain? Here's a very general rule: If you don't change out anything over several thousand miles (say, 3,000), you'll pretty-much have to replace all the driveline components (chain, cassette, rings, pulleys) at the same time, as the worn chain has worn the other components to match it. If, however, you change out the chain every 1,000 miles or so, the components thru which it passes will all last much longer, as they won't match the wear on a progressively-worn chain. Or did you know all that already?