Turnigy ? Fly Sky? Is the esc and motor Turnigy as well ? I have little faith in Turnigy gear.
You stated you were loosing control but you didn't loose it completely for 10-20 sec?
If the bec is not providing enough current under load then the servos would be sluggish but this would happen on the bench.
If the bec was not providing enough voltage then the receiver will/should brown-out and you loose control completely until the bec resets and then resets the receiver(several seconds) ....but again this should happen on the bench under a load test. Could be a bad motor or servos, or wiring too but this should all prove itself on the bench.
Loss of radio control at range is typically a radio/receiver issue.
If you can't trouble shoot by testing then you will have to start swapping components until the problem is corrected. Wiring, motor, esc, servos, etc...