Okay, everything checked out so far except the GP Relay. It appears it might be an intermittent old age problem. Old age....another issue entirely!
All 8 glow plugs check out between 1 & 2 ohms, I have 12v power to the GP relay by-passing the resistor bank. However, I did finally catch the GP Relay with my meter in hand, doing the exact same thing N8roro mentioned in a post about the "wait" light on and it'll crank but not start.
The GP relay was closing each time I turned on the ignition until about the 8th time. I only turned the key to listen for the click and I gave it time in between attempts. I did not leave the ignition on until the light went out. After I heard no click, I tapped the relay with my wrench, it closed and the "wait" light went out a few seconds later. It starts instantly if the "wait' light turns off rather quick. If the wait light stays on for an extended period of time before it goes out, it will not start but keep cranking until the batteries give up. I'm in AZ, it's not cold but apparently it's cold enough to keep it from starting without a functional GP relay.
Using the tech manual and an Ohm meter, I found the resistance between the two smaller wire studs on the GP relay was at 38 Ohm's. The tech manual says this measurement should be 6 Ohm's on the OEM. Whether or not the additional resistance is a big factor, the manual says to replace it. This relay looks to be the original; so, I ordered a GP relay and we'll see what happens from there.
Thanks for the assist CUCVRUS!
Hopefully, after the relay swap it'll start just fine for the long haul!
Scooter...