I wonder if those servos are overloading the battery pack during the hard manuever? Those servos are plenty strong, IIRC they're what I'm running in my fastest delta. I've got no experience with lipos and BECs, too high tech for me.

I do know one thing.......problems like this rarely straighten themselves out on their own. Consider yourself fortunate. I would get a big honkin' nicad or nimh pack and go with that. I would also exercise the controls while exerting pressure on the flaps. If you plug a voltmeter in while you check the controls under force, make sure the voltage isn't dipping below 4.8