BEC/Servo problem

[BrianG]

Even though a BEC utilizes low drop-out voltage components, you still need a solid 1v higher than your BEC output to operate. This is true for linear or most switching BECs (I've seen a couple designs that will boost or buck as needed). Failure to do this results in unstable BEC operation under load. And you can't verify with no BEC load because only under load does it get unstable.

Setting your BEC to 6.5v would require around 7.5v to work properly/reliably. You can maybe get away with as low as 7v, but that doesn't leave much room to account for battery voltage dips under load, which destabilizes the BEC.

I can with 99.5% confidence guarantee this is what you're experiencing.