So you narrowed it down the the UBEC. I have heard of some having interference issues with the CC UBEC, but the higher voltage making it worse is a new one. I'm suspecting that the higher voltage into the BEC is causing the BEC to work harder and therefore creating more RF noise and more interference, but I don't really know enough to make sense of that. One of the electronics wizzes around here can probably explain why that would be.
But, to start working on eliminating the problem. I would start with moving the BEC as far away from the Rx as possible and see if that helps. If it doesn't, you might try wrapping the UBEC in a few layers of aluminum foil. this might help shield any noise that its generating from getting out to the Rx. Beyond that I would say stick to the Rx pack, or get another BEC or something.
Do you have the Castle Creations USB programing wire? I would bump that voltage up to 6V if it were me. You will get more out of you servos that way, and maybe the higher voltage will help with the interference issue too. I don't really know
why it would, but it seems logical to me anyway... but its probably just me.