My guess is the connection between shaft A (the longer shaft) and the lower gear is loose. It could just be that the fasteners have come loose, or the gear might be cracked or stripped. It would probably be wise to get both a gear A and gear B, as eventually they'll probably both need replacement. I'd get the turbo shaft, too, while you're ordering as it makes the heli much easier to fly without fear of blade strikes or the flybar coming off.
Turbo Shaft:
http://www.rchelicoptershop.com/cata...che-p-342.html
Gear for shaft A:
http://www.rchelicoptershop.com/cata...in22-p-92.html
Shaft B plus gear:
http://www.rchelicoptershop.com/cata...s22-p-420.html
Harden Blades (definitely more durable):
http://www.rchelicoptershop.com/cata...che-p-169.html
I've ordered spares a couple of times from rchelicoptershop and they've been quick to ship and arrive. Of course, I'm in Australia so it might be different shipping to America.