The gear shifting on my commuter bike recently became a little random and, after checking chain wear, I realised it was time to replace it. However, the shifting is now worse than before and putting any significant load on the pedals in certain gears results in the chain slipping on the cassette (at least I think it's the cassette rather than chainrings). I've tinkered with the cable which is relatively new (8 months/2,500km) but that doesn't seem to have cured it. Is it likely to be the cassette that is worn and needs replaced or could it be something else? For reference, the cassette has never been replaced on this bike so has done 6,500km over the past 18 months including one Scottish winter. On a scale of 1 - 10 (10 being cleaning the bike after ever use and keeping it emaculate), I'd say the bike lives in a state of 5 or 6.
Thanks in advance,