Strange passthrough behavior

setting a gimbal with Copter 3.5.3 today. The issue I experience is jerkiness/signal drops on Aux channels, have set SERVO9-SERVO12 to 1 (passthrough). This translates into spikes in gimbal motion.
TX Futaba T8FGS, if gimbal connected to a separate RX, all motions are fine.
Tried, ferrite rings on signal cables, different power source for the gimbal, switching off each transmitter on board one by one and all together.
So basically - direct RX connection, works good, connected through PH2.1 as pass-through, jerks.
Any thoughts?

UPDATE. Any pixhawk output, not just passthrough, to gimbal controls brings in jerky movements (in basecam GUI they appear as signal spikes and drops). Switched Servo functions to 0 and commanded axes from Flight Data Servo menu.