Unfortunally Eco8 is such a fragile helicopter that the slightest misalignment on anything can be the source of your problems. I bought to ECO16 which is basically the same ECO8 with a larger booma and blades and could never make them pass that point, exactly as you describe.

The more I would advance throttle the more it would vibrate until one day I hit the boom with the blade and the whole thing broke apart, even the frame which is extremlly fragile. After that, no matter what I tried, and what I changed (that is why I bought the 2nd ECO16) it would always do the same.

My problems ended when I got a Logo10 instead, moved all the electronics from the ECO to the Logo and the very first try I could hover in no time. Now, 2 years later with more experience and knowledge I compare the 2 and realize what a bad helicopter the ECO8 is. I have read that the ECO Royal is somewhat better, but why bother, when there are so many other helicopters much better then the ECO.

One thing I can tell you is that the stock shafts, the main, the blade and the tail, all of them are extremely weak. Check them real good, make sure there is not the slightest bent on any of them, because they do bend almost for no reason. The tracking would also be the cause, but that is easy to check.

Good luck.