"Power issues. The motor is probably fine, but the ESC could be dying. if it's only a little IR one you're lucky to have had three years out of it. You could get a new controller board for it, but you might as well buy a whole new one."

"A further issue with large props is gyroscopic precession, which will resist deflection. So adding your four extra smaller props will improve the maneuverability, but not as much as you might expect. Still worth experimenting with though, go for it."