Thank you! The OPS is already relayed. One of the 3 big relays that I have on the firewall. Glow plugs, OPS an I've forgotten what the 3rd one serves. Exhaust brake, which no longer there. Regardless, the problem being intermittent won't make it easy. With my home-made t-cable bracket being in the way, my oversized paws, etc., I'll probably remove the air plenum (or whatever it's called) piggy-back a new wire to one of my spare (16) switched fuse terminals and install yet another toggle on the dash, to be ready for the next failure. All depends on IF, I can make it fail with a VOM hooked up solid at the fuel shut-off.
Have either of you ever heard of the shut-off, itself failing? Once accessed, I can easily exercise it, to check it out.
Anyhow, thank you for your patience, while I sit here thinking in black and white. What goes through my feeble brain, that isn't backed up by a hand-written note, is soon irretrievable.