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.