What you were saying about loosing power and then leaning it out makes me think that your running too lean. Does the engine run un-evenly? I suppose the situation is that your fuel wont make the power at the proper mixture setting, so you have to lean it out too much to fly. Then the engine starts to overheat and then looses power.

I'd say, do what darth said. A loose backplate caused me headaches for a while once... If you still have the problem after that, you really should try a diffrent fuel.

Good luck!
Rico