BBB Robotics Cape - driving TACTIC servo chatters

My setup is BBB Rev C with Robotics Cape, 11.1V 3S LiPo battery connected to DC Jack.
When I drive a TACTIC TSX45 servo, using rc_test_servos, the servo chatters and jumps around the position it should be driven to.
This same servo works without chattering when driven by a TACTIC TA325 3-chan RC receiver.

Interestingly, driving an Axial AE-5 ESC and motor from one of the servo ports works perfectly!

Any help and suggestions would be greatly appreciated.