When it’s over 75 degrees outside (which it is a lot in Florida where I’m at), the temp gauge will suddenly drop down to the cold range almost bottoming out. After a while it will come back to normal. This will continue to happen every so often, say every ten minutes or so. Only when I’m driving around town not when I’m on the expressway. Any clues to what this is?
I’d start by replacing the thermostat. It may be sticking when it opens.