Have you considered that you might have a bearing sticking on a motor causing the problem. I had 2 that froze up on me while in a hover and I was able to sit it down before it crashed. Worth thinking about
I bought the Arris 680 hexacopter and ordered some longer carbon arms to make it a 810 hexacopter. Right now I"m in the process of setting the gains on my naza v2 and that seems to be the only problem that I have at the time and even without the gains fine tuned it still flies great. I was...