Elevator channel twitching with Throttle input in all Stabilized flight modes

I checked the servo output, and it is not just the servo, the Elevator output actually starts moving and bounces around in every controlled flight mode (doesn’t happen in Manual or Acro).

Using ChibiOS with Pixhawk 1 board and PPM encoder with Spektrum DXE. Using Elevon setup, but I changed the servo output to Aileron/Elevator to further test, and it seems to be the Elevator channel only.

I also tested the system with a Turnigy 9X to eliminate the PPM encoder from being faulty. Same exact issue. Switch to Manual and the issue goes away, but in every other flight mode (save Acro) the servos jitter with any throttle input.