"Part of the package I brought was a copy of Mach3"

What was the package and where did it come from? There are quite a lot of flaky pirate copies of Mach3 around that kind-of seem to work. Except that they are riddled with bugs and inclined to fail in various unexpected ways. Do you have a genuine Mach3 licenced copy? This isn't meant as any kind of criticism - if you bought something that says it has a copy of Mach3 it's reasonable to assume that that's what you are getting. However, it isn't always true and it might just be the reason you are seeing peculiar behaviour from Mach3.