How are you generating your gcode for the test circles? A common problem is not using constant-velocity settings for curves. If your gcode is coming out with a series of straight line segment approximations to a curve, and Mach3 is set to follow these exactly, it stops the machine at each "corner" and then restarts in the new direction. Result is vibration/juddering. Set it to "constant velocity" and it will approximate the straight line segments by rounding the corners very slightly but it keeps the machine running steadily. Even better is to get your CAM programme to generate arcs rather than straight-line approximations and Mach3 can do a better job still.