My money is on the PC being the problem and I would swap that first, think if you had a noise issue the symptoms would be more constant. Watching the video and what you can do to get it working again suggests to me it's some sort of buffering problem, when you send a keystroke it could be clearing that, the minute you release, the other tools start sending signals and off she go's again.

What's strange is how it doesn't do it when cutting air...hummm

What hardware is existing from the previous machine.

.Me