Miskin,
Did you use the stock plastic control horn clevis that came with the kit? If so did you notice if they had come off the control horns when you picked up the parts?
I had two scary flutter incidents of the tail before I changed to Metal Sullivan clevises and tightened the cables. Also upped the servo voltage. No sign of flutter since.
Remember make your cables nice and taught like a guitar to help prevent any flutter and absoultey DO NOT FLY FAST. No more than half throttle if level should keep it safe. The hype is more of a stringless kite. Flops and spins like a fish out of water it is not meant to fly like a P51.