In the past, I’ve had a problem with the flat cable running from the Bitrunner to the Spindle Enable switch. When the router should come on, it simply didn’t. So far, I’ve been lucky enough to catch it before damage was done, hitting Pause and then Stop. Wiggling the connector at the Spindle Enable seems to cure in - sometimes. Eventually, I used a zip tie to hold it in an odd position and that worked.
I just finished building an enclosure for my Shapeoko Pro, which required disconnecting the Bitrunner. And of course on the first small job, the router failed to start. After half a dozen attempts, I finally hit the sweet spot with the Spindle Enable ribbon cable connector. But I’m concerned that it will fail in mid job, ruining any number of things.
Talk to support and get a new cable. If under warranty it will be free. If out of warranty you will have to buy it but i cannot imagine it will be too expensive.
I would try corrosion block product in the connectors. Sometimes a thin layer of oxidation can cause this. Even WD40 would help trouble shoot the issue but will not leave a long term film like corrosion block n
It will be a process of elimination. The cable is easiest and likely cheapest. Intermittent connections are hard to diagnose. If your problem was 100% it would easier but since it is only intermittent???
To answer your question about which end is the problem the answer would be a crystal ball or Ouiji Board. I worked on every type of business machine made and intermittent problems are the hardest to figure out. I know it is hard to hear but it will just be a process of elimination. However if you fire up the parts cannon every part you replace can introduce new or different problems. So be methodical and do one thing at a time. Firing a parts cannon can be costly and lead to new problems.