I cannot fathom the fan clutch being the culprit, since I cannot figure out how it would ever engage with a 20F. ambient temperature............the fan does stop immediately when the keyswitch is shut down..........even in very cold temps..........FWIW.

Well, kinda out of suspects, then, I think ...

If coolant pump is letting pressure escape will usually show leakage through weep hole.
If the coolant system isn't holding pressure?
If there aren't any blockages?
If there aren't any "air bubbles" - this can cause weird things to happen ...
If there aren't any issues with the radiator - clogged, blockage moving around - blocked fins doesn't sound like an issue ... consider cleaning it if you haven't ..

Based on what you have said, I would:
1. Ensure there are not air bubbles in the system (let it run and open hoses in a couple of places to bleed
2. Make sure system is holding pressure - cap OK? Leak or hole in the reservoir?

__________________
George Stephenson
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg