The problem with our cars is that the upper Rad hose and engine are higher than the rad and since water seeks it's own level, (meaning that the lowest point that water sits is also the highest point in the system) and you end up with air in the system. This air insulates the thermostat and keeps it from opening. The times you here the car boil over is the time you need to put water back into the system. Every time it pukes water out it's lost and just puts more air in teh system.
You need to start with a cold engine. Remove the bolt to the right of the water neck. It's the one with the plastic thing that reads replace B4 starting engine or something like that. Be sure to use a backup wrench to keep from twisting the tube. Add coolant to the overflow.
Add coolant until it comes out of the top of the tube. Replace the Rad cap and bolt, start the engine and let it warm up, drive it around the block with some spirit to get the temp up, come home and shut it off. Let it cool totally and add coolant to the overflow as needed. Keep a watch on it for a few weeks.
If you keep driving it like this you WILL blow a headgasket no if's and's or but's about it. I can't stress this enough.
Jeff