I would bet that the .51 and the 10x6 prop is the culprit. try an 11x4, 12x4 or 12.25x3.75 prop, that 10x6 at 12000rpm is producing about a 70 mph pitch speed, where most of the people flying this plane are using a 4 inch pitch prop maxed around 10000 rpm, which gives you around a 40 mph pitch speed. If your elevator linkages arent perfectly slop free, and your servos are sloppy around center, maybe combined with a non sealed hinge gap and no reinforcements, anything over 35-40mph would probably introduce flutter, and rip the elevator off. by changing to a bigger, lower pitched prop you gain alot of thrust and lose the speed~which this plane was designed for.

Half throttle may be producing a higher speed than you think with that 10x6.

Dylan