I've just got a similar one myself.
I had problems with movement on mine as well, it could be one of several problems. The interface between the computer Industry assume that the Cnc uses a parallel port as a connection. Forget about a standard usb to parallel converter at a few quid, if it is a laptop then a picmca is not much better it all has to do with timing, I don't fully understand I just know both method failed on me even a proper parallel port can cause problems it seems the cheapest option though not that cheap is the uc100 it acts like a usb to parallel port adaptor but has a lot more in it mainly its own timing chip so information just needs to be sent to it then it sends the data out in batches using the timer chip to send it out at the correct time.
kendrick