Like I said in post #17, set f5 to Aperture ring to control the aperture mechanically with the ring instead of electronically with the dial to get the most consistent exposures.
When you shoot wide open, the aperture will be absolutely locked and can't get anywhere. This would eliminate the aperture as a factor for the flicker.
The arm on the lens looks all right, have you looked at the arm in the body? Have you tested the lens for a sticky aperture yet?