Temp gauge mystery

I've had 2 cars (71 Demon 318) and my now 73 340 that the temp gauge would swing rapidly from 1/3-1/2 of the normal gauge to all way hot to back to normal in matter of seconds. I sold the Demon way back in the day. For the 73 - I installed a Autometer eclectically gauge and it runs at 185-190 all day and is connect along with the factory gauge which barely gets to the first normal line. I have since changed the radiator when the temp gauge was swinging cause the previous owner had an undersized aftermarket installed in it.