AusTai

My 10-year-old mountain bike (Gary Fisher Tassajara) would occasionally jump a gear, especially during heavy stomping, so I bought a new chain. I also thoroughly cleaned the front and rear derailleurs, the front chain rings and rear cassette. The bike sounds a lot quieter now and in casual riding it works well enough. But on a long ride tonight (Austin's trail of lights), there were several frustrating moments when I wanted to accelerate hard and suddenly the rear derailleur popped a gear. grrrr...

I haven't looked at the bike closely since but before the ride the alignment of the rear derailleur looked correct and, as I said, the drivetrain noise was very minimal.

Is my rear cassette shot?

Thanks in advance

Maelstrom 

You changed everything BUT the cassette? Thats your problem this time.

seely

Is your derailleur hanger bent? You should really always replace the cassette and the chain at the same time as they do wear together.

cryogenic

bent derailleur hangers can definitely cause such things... wouldn't surprise me if it was slightly out of line... I've never crashed mine and mine was barely bent causing sluggish shifting. Turns out my bike had fallen over and landed on that side once when my cats knocked it over. Re-aligning the hanger fixed the problem.

Mtnbike247365

You can check your cable tension, see if it is too lose or too tight works for me.