I assume you're using 50/50 coolant/water mix.
The cooling systems on these cars are quite marginal. In hotter climates, they tend to get very unhappy (ask me how I know).
Your radiator may be clogged. It could be time to pull it and have it boiled and/or rodded.
Your water pump may be corroded. (You know what to do here.)
However, none of this addresses the boiling problem. Often, the expansion tank itself is the culprit for leaking/losing pressure. So after all the advice I gave you, perhaps a new tank is in order.
Any way you slice it, you're going to have many new cooling system components before you're done. But consider this to be an excellent long-term investment in your car's health -- because you will be replacing this stuff either now or later!
