I put about 900 miles on a Real cassette last year. It was never the snappiest shifting cassette, but it always ran fairly smoothly. I just replaced my KMZ chain with a SRAM PC-99 hollow pin, now the middle of my cassette skips. I used mostly the middle of my cassette commuting the three middle cogs saw the most use, now thats where it skips. I think it's time for a new cassette.