It may very well be a bec issue, as the servo draws heavy current the bec inside the esc shuts down for a second, and if combined with spectrum causes a brief moment of failsafe
One cheap way to test is by either fitting a capacitor into a spare channel, or fitting a separate rx battery into the receiver, but make sure to remove the RED wire from the esc to rx wire on the plug first.
If either of those fix it, chances are its the esc's bec system. These are very common to fail and when good only provide 2.5-3.0 amps at best. I'd personally recommend replacing with the mamba external bec as its capable of handling 7amps all day which will power the thirstiest of servos.
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