It may be the USB cable. In another forum, the problem of frequently losing connection with the computer was found to be external noise from overhead light fixture, and a high quality USB cable with double shielding was the solution. They had a video that showed the action with two types of cable, and switching the light fixture on and off.
Also, many have found that the Ethernet connection to be far more reliable than the USB, in the controllers that make both types, like the Smooth Stepper.
Noise problems can be very elusive, but have been one of the most frequent problems on the Mach3 forum.
John