When you say the "computer" locks up - do you mean the computer, or is it Mach that locks up.
Check your GCode - or try running something so simple that you can see it is right.
I have been writing GCode over the last week, and the favorite thing that locks up Mach is silly errors in the code.
This is particularly apparent when using subroutines, if you have misnamed it, or not put in a return command.
Errors usually get picked up as you enter the program. Mach runs through the entire program and gives you the tool path pciture. Do you get as far as that, without anything appearing in the comments window.
I take it, other than this, everything else works fine, like jogging, accuracy etc etc.