I had a similar problem on my X400 Pro, (upgraded to basicaly a T-Rex 450XL) it would spin up, I could hover and every so often the power would surge then cut and the heli would drop. I ran it up while holding the heli and spun it up and down, or holding constant revs - and it was solid. Finally I put the heli on a smooth large table where it could slide and spin around with the tail and ran it up and just held it below a lift-off-hover and then it cut out again. (so I thought that this really looks like EMI, electrical-magnetic-interference) so I removed the ESC connector from reciever and wrapped it thru a ferrite donut about 10 times. (I used a 33uH M9863-ND from digi-key.com and removed the existing mag wire first) and now it's as steady as a rock - even with the flight battery mostly discharged.