...I dont think its a brown-out reset, because the arduino doesnt hit the bootloader... it goes straight to the beginning. Not only that, but I have a random movement tester that works 100% fine with the boards... no resets. (also, the bug happens with the stepper boards unconnected!)
note to self: i will not enable interrupts i do not fully understand!
P.S. Too bad I didn't find the fix for you tho, would've liked a RepRap printed item--particularly if it was another RepRap. :-)