Is it actually boiling over, or is the gauge just indicating "hot"? Try using an infra-red thermometer to confirm that the coolant is actually getting close to the boiling point. The most common cause for overheating is exhaust gas getting into the coolant, from either a cracked head or a leaky gasket. A dirty radiator is another possibility.
If there is a shop nearby with a 4-gas exhaust analyzer, ask their diagnostic tech to hold the exhaust sampling probe over the radiator filler neck with the cap removed. If the "hydrocarbon" reading goes up dramatically, it's a sure sign that exhaust is getting into the coolant. Just be sure NOT to get the probe down to the level of the coolant- - -it really does nasty things to expensive internal parts if antifreeze gets sucked up into the machine!
Jerry
