Hi,
the fact that its random and happens on tried an trued Gcode files suggests electrical noise to me.
The UC100 is a great little controller but is subject to electrical interference. This is a recurring issue with USB connected
controllers.
In the first instance use a short, short as possible USB lead of the best quality you can find. Install ferrite split rings, one at each end and one
in the middle.
You should run your spindle through a GOOD line filter at a minimum and a line reactor a best.
Additionally it sometimes helps to draw AC supply for your spindle from a different AC circuit then the AC supply for the PC, controller power supplies etc.
This type of electrical noise is called 'conducted noise' an is often overlooked by CNCers. Line reactors are the best remedy for spindle induced
conducted noise.
Craig