The Mach3 software tends to operate at a low level and on older machines its biggest competitior for resources can be the graphics especially with onboard or lower spec GPU that can grab memory fo compensate. I have seen similar problems and sorted them by simply reducing graphics overheads you don't need full 32bit graphics for mach3 and reducing graphics overheads can free a large amount of resources on older machines, but worst offender is if you enable Mach3 hi res screens options it will then draw screens twice to remove pixelation.