Sounds like it's time for a major service of the system. New hoses all round along with new radiator cap, plus expansion bottle cap if you have one, and a new correct thermostat should bring things back to normal. Once you start leaking coolant when hot, air is sucked back in as it cools. Air in the system makes it all worse.
Your gauge problem can only be solved by replacing the whole system from dash gauge to head sensor as they are a sealed unit. Have a look at the list, there is a thread on this within a few posts.