Hi Frankg521,
Mach4 is quite demanding on a PC, particularly if that PC is required to provide the pulse train for Darwin operation. Some PC's can handle it and some can't so the best all-round solution is to use an external motion controller. OK it's a bit more expense but, you will find, it is well worth it in the long run.
There are many Mach4 motion controllers for you to choose from. I started out using Darwin now I use an ESS on one machine and a 57CNC on another, it is just a matter of choice (and of course, selecting one that offers good customer support).
Tweakie.