for the 24volts are you using the original transformer/ big cap in the boxford or a switch mode?
switch mode psu can trip with a no load as well as full load condition

re breakout: i used a cnc4pc buffered breakout and a step to analogue board from jon at cncdoctor.co.uk (always on ebay)
if your going to run off a laptop make sure you use a buffered board to pull the 3.3 v signals to 5v

the gemini is a torque drive so wants a max 8volt analogue signal not 10volts.

the gemini is pretty crap at holding correct rpm. and takes a hell of a lot of tuning to get to a reasonable state.
i hate steppers so i put in mitsubishi servos in mine. making it rapid at 6m/minute is fun to watch on such a small machine.

mach 3 will do whatever you want. its totally configurable when you start digging a bit deeper into it as ive proved with my latest conversion
(wood machining centre with 2 routers, 24 vertical drills, 10 horizontal drills all offset from each other)