[RC-Monster Mike]

Running a receiver battery won't tell you if the servo is drawing excess current - the problem may seem to go away, as the load will no longer be on the ESC BEC, but the root cause may still exist. My guess is that the servo is drawing excess current, as mentioned a couple times in this thread - dropping in a good servo will fix the issue. Also mentioned in the thread - you get what you pay for with servos for the most part. You don't need a super powerful servo, either - there is no throttle return spring to fight. :)