Lucan07 your wrong about mach3 not needing much graphics power. Infact it's one of the main reasons people have trouble with mach3 when using PP or lower spec PC's. Just clicking in toolpath screen would lock mach up on some PC's with poor or low graphics. This Big area where mach4 as improved.
While External controllers do take lot of the work load off Mach3 it can and does still happen with them. Even the Cslabs do it albeit to much lesser degree.

Charlie While I think it's more PC related just for precautions sake I'd go over the wiring and check for lose connections or any nipped cables etc.