Stock as in EPS300C, C gear ratio? There is also an EPS350C and an EPS400 powered Slow Stick.

There might be a high-resistance connection in the battery pack, or the cells are just crappy Chinese knock-offs. NiMH packs will get better after a few cycles, though. It may be that your packs just need to be charged and discharged a few times.

7-cell and 8-cell packs are much more common on Slow Sticks, but 6-cells should give you adequate power, if you get decent batteries.