water temp light blinking

[zetan]

1992 VW Jetta 1.8

When cold, the motor starts but runs really rough. It dies if you put it in gear (auto). The water temp light starts flashing and the temp gauge is pegged far right (like a missing ground). After warming up, the light goes away, the gauge starts working (little less than 1/2), and it runs smooth.

I checked and cleaned the connections on the upper & lower water temp sensors, 3 prong sensor on the radiator, and water level sensor.
Unplugging each while the motor was on (and warmed up) did not produce the flashing light.

Any ideas how to diagnose the flashing water temp light?