Gear Jumping

[okeja]

Like already mentioned, you are on right track with new cables
...or at least pull each casing out of the stop, slide it up the cable then add some grease (lightly) to the cable where the casing will be
if the section that loops from the frame to the rear der has a stop attached to the seat stay (part of frame that runs from rear wheel to seatpost), water/dirt/gunk will often run down the cable & into the housing which can lead to trouble - big benefit from cleaning/greasing this section


Cables are important
The shifter is the indexed (digital) part, while the ders are "analog"; the cable transfers shifter movement to the ders ...so if the cables are dirty, worn, binding in casings, etc - it is possible to get poor shifting or gear jumping

Cable Casings are also important as they allow the cable to transfer movement around a curve
If the casings are inflexible or if the liners are worn - it can lead to poor shifting

[freddyb]

I had similar issues on my first ride of the season...jumping around between gears on climbs. Also had problems getting up into the climbing gear because it kept wanting to jump down a cog. I followed the guide here: http://www.utahmountainbiking.com/fix/rshf-tun.htm

Turned out the cable had too much slack and was messing everything up, so I took out the slack, followed the adjustment procedure, and it's shifting great now.

Hope this helps.

[tclay]

Just finished a fight with a similar situation, had no problems shifting but if I hit any bumps under a good amount of torque on the cranks or put too much torque while climbing my chain would either jump down a ring or off my crankset to the outside or it'd pop and skip and I'd lose all momentum. Ended up replacing chain, then cassette, and then the big and middle ring, haven't had the issue in the two rides since the last replacement of the crankset rings.

[Di_bear]

Just finished a fight with a similar situation, had no problems shifting but if I hit any bumps under a good amount of torque on the cranks or put too much torque while climbing my chain would either jump down a ring or off my crankset to the outside or it'd pop and skip and I'd lose all momentum. Ended up replacing chain, then cassette, and then the big and middle ring, haven't had the issue in the two rides since the last replacement of the crankset rings.

I had that problem with my new rings and derailleur. It was fixed by adjusting the limit screws and doing some fine-tuning. The rear, well, still working on it, but it's a lot better than it was.

[trout_smith]

A bent hanger has been the culprit on plenty of bikes I have worked on, especially if you can't seem to fine tune it and it always seems a little off. This isn't something you can tell just by looking at it. A bike shop should have a tool and it takes about a minute to tell if the hanger is bent.

Or, as others have said, mismatched (uneven wear) cassette and chain, sticky cables/housing and incorrect chain length could also be causing mis-shifts.