OK. Unfortunately the load that your meter uses is really not sufficient to properly test your packs. From the specs for that meter:
Simulated Load on Receiver side is 225 to 275mA
If you were reading 5 volts with a 225 to 275 mA load I can almost guarantee you that you experienced a low voltage failfsafe event. Four servos moving under the flight loads you were seeing during the mild aerobatics more than likely caused the voltage to drop below the 3.8 volt battery failsafe point. Worse yet if there is any biding on the linkage at the extremes of throw you pull even more amps and drop the voltage even more. I'll bet if you think about it that you dropped your throttle to full low and up again and that regaining control was pretty much coincident with doing that.
I would put a good quality 5-cell NiMH pack in there so you have about the same weight and capacity. Then get a good meter that puts a more real life load on your packs. I teat all my packs at 1 amp load. When I get 5.9 volts on a 6 volt pack I stop flying and charge.
Here's a great source for battery packs:
http://www.hangtimes.com