I stopped by my VW shop at lunch to pick up a thermostat and 0-ring. I
told them what was happening... hot coolant in the rear, cool radiator,
but water pump working well enough to pump coolant to the top of the
tank, sitting level.

The mechanic said it was probably a stopped up radiator, for which
there is no cure but to replace the radiator. Anybody have experience
with this? Does this sound reasonable, and if it does (symptomatically,
it sounds reasonable, but I mean can they not be cleared?) who has a
used radiator (single sensor/switch, low driver's side)?

thanks,

Jim