If it helps I've had a similar problem for many years on my 84 GL.
Everything is running just fine and, all of a sudden, I have a flashing red
light on the temp gauge. I comes and goes. When it happens  I just check
the mirrors for white clouds of coolant vapor behind the van. There's never
anything. It's an electrical thing in my case. My diagnosis says it's in
the wiring not the gauge pack or the sensor. So I just check the coolant
level occasionaly live with it. But then, I'm lazy.

Tom Brobst