RE: DD control problems
Usually, a BECs amperage rating is reduced for higher voltage. The rated voltage is usually at 7.4v input. Likely you are only getting 3-4 amps peak with 3 or less continuous. This is probably not enough for those high power servos.
I run the Castle Creations 10 amp BEC with a 1000 mAh lipo on all my planes without issue. 5-8 flights is well within it's capability without re-charge.