Copter takeoff with althold mode have unusually flight

I have a trouble with the aircraft recently. I use the copter-3.3.3 and F450 rack.
issue :
I takeoff my aircraft with althold mode ,when I pull up the throttle stick(above 50%) ,the aircraft takeoff immediately,
But it drop down suddenly,Unless I pull up more the throttle stick the aircraft will keep the altitude.

  1. When aircraft close to the ground (about 30cm) will climb up and then drop down,repeat this action .when I pull up the throttle stick,it climb up 50cm ,will be normal.can keep the altitude.
    2016-07-19 19-24-27.tlog (227.3 KB)

250.dwg (74.1 KB)