On my '95 Ciera, the temperature gauge will be reading normal (middle of range,) then will suddenly bounce very rapidly back and forth toward the hot end of the range. After a couple of seconds, it will return to reading normal.
I noticed the same thing happening the other day after I had only driven a mile or two and the engine wasn’t yet up to normal temp…
I’m guessing that it may be a malfunctioning temp sensor, or could it be just a bad electrical connection?
Any other ideas? Your input is appreciated. 
Check the coolant level in the radiator (when it’s cold), then clean and tighten the connection at the the temp sensor. If that doesn’t work, try a new sensor.
Coolant level is OK. I’ll check the commection when it stops raining.
Thanks!