Sorry for asking the obvious but you bought this as a ready to run unit? If so has it run ok from the start and then the problem arose or has it done this from day one? Have you done any alterations to it? Any details of the electronics (stepper drivers,PSU, etc) how is it connected to PC or is a built in unit? The more you can show and tell the better chance someone may find the problem. Another check the forum guys can do is look at your XML file, copy it from the Mach folder and post it on here.
Stewart