RE: Is anyone experiencing interference ?
your esc may not like your using 4 servos, disconnect one of the other servos and see if the elevator servo now works, another thought would be to use a ferrite ring
also try to disable your bec temporarily by removing the positive wire from the plug to receiver end, this can be done with a very small screw driver. the wire can be reinstalled later. after disabling the bec now plug a separate 4.8 volt battery to the receiver. this will allow you see if the esc is actually not capable of handling all 4 of your servos.