Brownouts are generally caused by not setting the end points correctly, are you doing this with the wheels on the ground or in the air?

If the problem persists after ensuring the EPA is properly set, also explore various frame rate settings with your radio, that can also cause brown outs.

As a last resort I would consider installing a UBEC to bypass the internal BEC, some have reported limited success with a glitch buster, but since a UBEC is about the same price, it would be a fool proof solution.