Temperature gauge does not work on my 1970 340 Duster ?

I had the same issues with my temp gauge, I could never get it to work. I checked all grounds, heated sensor outside of car, replaced voltage limiter, it all tested good, still gauge did not work. I gave up on it, and just drove the car wit no temp. gauge. Eventually, it started working on its own, it took a while for the gauge to move, but it did work. The only thing I could come up with was low coolant, an air bubble, or something like that. After flushing the radiator and block, it seems to works fine now.