I just rebuilt my t-rex 450, I've been working on this for about 2 weeks chasing whatever is making the tail shake, I just can't figure it out...

I've replaced:
main shaft
tail shaft
front pulley for tail
rear pulley for tail
flybar
main gear
feathering shaft
blades
flybar paddles

just about everything I can think of, I even took the tail apart to make sure the belt wasn't crossed, yet I can't figure out what has gone wrong... does anyone have any good ideas?