I've had a couple 375fps+ M4's and no nimh batteries could properly cycle the heavy springs I was running. Trying to crank the spring on a nimh would usually get 1-2 cycles max and or stop in the middle of one. Of course the motor would still run fine when powered by the little battery, but a NICAD was the only thing that would produce the juice to run my guns normally.
Try a large NiCd battery, the max you can get your hands on.
I'm building an MP5 around a Systema Magnum 170 set, and I'll need a 10—12v for that... but I'll be downgrading it 400fps to safely shoot my pals