I'm wondering if it's the hardware at issue, that ebay kit is a really badly matched kit I suspect. The steppers are huge and have a massive inductance value, the ball park PSU they need to perform optimally is around 150VDC.
How big and heavy is this machine of yours?
More knowledgeable people will chime in I'm sure, but I think the response and speeds/rapids will be pretty bad with this setup?
cheers, Ian