Used to have the same problem with my walkera 37, you need a battery with a higher C rating. I would recommend at least 20 if not 25C. You are most likely having voltage dropouts. Also, I would recommend a separate BEC like the sport BEC from Dimension Engineering, or one from castle creations. Only if your servos are a little slow or your ESC runs hot. Happy flying
That post was over 2 years ago, but his problem for one is he's using way to big of a battery for that size of a heli along with that dinky esc. A 4000 mah 3s lipo is just way to much, and is a very high disc-loading for that heli. He most likely found that out.....
