i bought a 2010 gary fisher mamba back in august and within a couple weeks the chain started slipping. when i pedal hard (even sitting down) it slipps a couple teeth. i took it back to the shop and the guy adjusted something that affects how much of the chain is touching the rear gears at any given time. it worked ok for like a day then started again. ive tried adjusting it myself, but if i adjust it too much, it wont go into some gears, but if i losen it it goes back to slipping. what can i do?