Thanks for the info, you all. No more blinking or false high temp now.
I removed, cleaned (there was crud on the inside-the-coolant-tank contacts), replaced, and tested the sensor. Specs are 31k-65k Ohms with tank full of 50/50 coolant/water: (http://www.vanagonparts.com/tech_bulletins.html#013 #90 92-03) 
Mine's at 44k Ohms. It's probably the original sensor from the factory. 

I think the problem began bcse Cunegonde is usually parked pointing forward steeply downhill. If I filled the coolant tank completely, and the overflow (behind the license plate) tank was also at max, it would all overflow when the van was parked on the flat, much less facing the other way. So the coolant tank was chronically low. Hence sensor electrodes with dried crud.  I check the cooling system every time I open the rear hatch, since my very first mechanical experience with the van was a failed water-pump engine overheat. Now, I'll just keep the overflow (license plate access) tank on the low side, but the main tank at max.