Bicycle Mechanics - rear derailleur shifting on it's own

I am having trouble with my rear derailleur on my mtb. Whenever I go up hills and put a lot of force on the pedals it seems to just start shifting on it's own. I brought it into my LBS and they adjusted the cable and said it should be fine. However, on the next ride it did the same thing. Is this a problem with the cable tension or is there something wrong with the rear derailleur? Any help would be appreciated.

Thanks,
Ben

Off the top of my head, my guess would be that the dérailleur and shifter need a bit of adjusting. Depending on how the parts are built, there will be barrel adjusters at both the derailleur and at the shifter. By tightening and loosening those, you pull the derailleur one way or the other so that it more precisely rides on the cogs.

I know that with some cheap indexed shifting systems, it is next to impossible to get it calibrated perfectly - either it will shift up well, or shift down well. Getting it to do both and stay put when in gear gets to be difficult.

Anyway, that's what my first guess would be. But see what others have to say as well.

If a cable tension adjustment doesn't fix it then you may have a bent derailleur hanger. The shop should have checked for this.

Al

also check the chain wear.
worn out chain can exaggerate minor issues with the cable or derailer.

Could be sticky cables and/or the guide under the bottom bracket, too.

See Sheldon Brown's article on "autoshifting". http://www.sheldonbrown.com/autoshift.html

Al

Have you replaced the housing? Sometimes old housing allows enough friction to throw off adjustments.

See Sheldon Brown's article on "autoshifting". http://www.sheldonbrown.com/autoshift.html

Al

+1. I stumbled across this tonight looking for info on something completely different, and immediately though of this thread.