Bought a GT ZuM 5.0 a while ago and got some awesome riding in during the spring/summers. Recently moved to cali from tennessee and didn't ride it at all until now. Whenever I'm in the top gear (8), it seems like the chain will just slip off one of the teeth for about half a pedal turn. This occurs both on a hard pedaling uphill and even level ground with hardly any resistance pedaling. Checked the cassette and I can't really see anything noticeably damaged but I'm pretty noob at bicycle maintenance.

I remember back when I was in shape, I never downshifted from 8th no matter how slow or how steep the hills were...could this extra tension have caused too much stress and damaged the cassette/chain? Anything I could do to fix it (it's really annoying to ride in 7th -_-...)? Thanks guys!