Due to the predictability you can pretty much rule out interference. Sounds to me like the characteristics of a stall. I don't know enough about the Venus to know if it can stall on knife-edge, but that just sounds like a stall behavior. It's also possible that the rudder servo is putting too much demand on the system and causing a temporary low voltage condition that can cause problems depending on what type of system you're on (PPM, PCM, or Spread Spectrum).