Ardupilot antenna tracker yaw Oscillation

i have a antenna tracker with ardupilot tracker 4.6 firmware installed in cube orange and here3 gps, using 2 servos 180,90 degrees for yaw and pitch.

when i set mode to scan after arming it only moves the pitch back and forth and fast oscillation of yaw, even if i connect it to simulation through mavlink out using micro usb at a certain point yaw is oscillating.

attaching my log
1 01-01-1980 05-30-00.bin (620 KB)

@rmackay9
look at this