Bios needs to be set to ECP. . . Also are there any settings in the bios for any type of power managment on the fly CPU clock frequency changing type settings, will say something like EIST or C1E.? If so then disable them. 99% sure this isn't your problem now but it would be when you did get it sorted because Mach doesnt like the CPU being messed with while working.!!

Would completly eliminate the PC by try connecting to another (But not Laptop).
Also forgot to mention make sure the PC doesn't have Apple Quicktime or any Adobe products installed. Turn of any power management, screen savers etc and auto updates.

Few only took 30mins and 6 attempts to write that so must be better now, think I'll try and finish that welding now.