It's probably resonance, like I mentioned before. A lot of people running that combo have similar problems.
Make sure the pulse width is set to 2 or 3 in the motor tuning. Run the driver test and make sure you get good results. Make sure the screws are not binding at all. You could also try changing to 1/4 or 1/2 stepping and see if that helps.
As was mentioned, you can try Mach2, and it may work better for you on the slower PC.