Grounding of the computer.... well, that is an interesting one, in that I'm not entirely sure how to check that or where to perform it. My cabinet is an industrial one that has an internal mounting plate. My 'computer' is a motherboard that is mounted the the plate (using standoffs in the correct locations, of course). The power supply was mechanically mounted to the same plate. Finally, an American style 120VAC plug was also mounted to the sheet. The power to the latter is switched elsewhere in the circuit. Grounding for the plug goes to a central point (although American plugs also ground through the metal box, which as I said, is mechanically grounded to the sheet.
I don't know of a normal place to attach a ground to a computer. Normally it gets its ground through the plug.
You may be onto something though.... I pulled out my COR relay, which means that the Vacc, and anything it supplies, would not be relevant. When I press the START button then the pendant freezes (this is a newer discovery). I'm going to go back and try that test with the simulator when I get a chance... But it definitely seems like a pattern of negative interaction with the START switch is at play here. I've confirmed the wiring but will do so again.
The start and estop circuits are provided as JPG attachments in an earlier post.
Update:
You may be onto something. I got the simulator to hang once when I pressed START (no COR relay in place). But then it all started working. I went back to the setup I had been using last night, where the failure after pressing START was 100% predictable, and now pressing START has no negative effect. Argh! !%^&^^#@#!@#@$% This is the stuff that can drive someone nuts! Spurious problems suck.

That said, grounding issues are right up the alley for problems like that.
Power supply design:
I should describe power supply design... I'm pulling all of my low voltage power from the computer power supply. The only (indirect) exception is my 24VDC, which is a small buck board connected to the power supply's 12V. No particular reason for posting the PS information other than it may twiddle someone's thoughts...