That's really weird that your batteries work fine in your friend's Blackout XTE (Pro brushless version I am assuming). Is it possible you got two bad speed controls with faulty/over sensitive low voltage cutoff? In a situation like this, I'd turn off the ESC's LVC, and use a little
LiPo low voltage alarm to test things out. You can find those LiPo buzzers/alarms for a couple of dollars each. I'd set up the low voltage buzzer for 3.4v/cell to see if your battery packs are "up to par".
I'm sure you are balance charging the LiPo packs fully. Right?