Take a piece of paper and hold it out your car window while you're driving. That's flutter. It is speed dependent and may be a moron flying a plane in a way not intended and may be a poor quality kit, but more often than not it's a sloppy control setup or flexible controls. Actually, the misusing the plane thing is pretty common too.

I'm one of those morons who likes to put my planes in terminal velocity power dives to see what happens. I've lost two elevators because of that. My controls are much better now and I still do the power dives, but I don't lose the planes any more.