See if your mainshaft slipped. Ya dont want any up/down movement. Maybe the collar above the bearing tower moved. Hey, wind will do it too !!! Man, sometimes i'll be hovering, and all of a sudden the heli will clime 10 feet on me... But I have also had the mainshaft move and that will make it hop too. Also, these heli's have alot of slop in the pitch, so you may want a little positive pitch (maybe 2 degree's) at throttle down. Be sure to lift up on the control tray when checking the pitch, or you will have a ton of negetive pitch when your done, and not know it. That just simulates the heli hanging in the air by the blades. I bet my BCP has 5 degree's of slop in the blade pitch !!! No Problem, just lift up on the control tray, then take your measurment.