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The next check is Junction 177A-B-C in the front panel.  If power is there then next is the top terminal of the rear start switch.  If no power is there then there is a wire problem between the front and rear, IMHO.  I have replaced the start switch previously.  It has been intermittent for a couple of years, sometimes months apart.

Newell supplied the diagram yesterday.

Solid plan, split half method, start in the middle and work towards the end that is showing the issue. Since your issue is intermittent, you may think about wiring in a temporary light to the 177A-B-C terminal so that you can spot it readily when it happens. Is there an easy way to run something up to the drivers area to keep an eye on it? 

I notice there is a 5 pin plug near the start switch, is that the plug for the start switch or a separate plug? That may be worth checking. I am not sure about what kind of plugs Newell is using but sometimes contacts in plugs can be an issue. 

FYI, I am an industrial electrician, I have always loved troubleshooting difficult problems and finding solutions. 

Glad that Newell provided you the diagrams!