Wobbly Cassette

Lord Jimmy 7th
Lord Jimmy 7th Posts: 379
edited October 2009 in Workshop
OK probably a bit of a dumb question but on my trek my gears are not perfect, the chain occasionally changes of its own accord and seems to 'snag' when i look down at the cassette it doesn't rotate in a perfect circle, which on my mountain/hybrid bike it does do?
Does anybody else's cassette wobble or is it just mine?