I recently bought mach 3 and a Fireball cnc maching with motors from Probotix.  I am having trouble getting the motors to run.  
If I start up windows and fire up mach 3 right away and jog the motors they run perfectly and nice and smooth.  Then after a little while they start making alot of noise and move in jerky steps.  I tried going through the windows xp setup on the machsupport website and it still acts weird.
I am running it on a desktop computer with a 1.8ghz AMD Athlon XP 2200 processor and 1GB of RAM.