I'm an old diesel mechanic. I would have the local auto parts store pressure check the cooling system. It could be something as easy as a $4 radiator cap. If the system holds pressure, my next step would be to check the radiator. It's fairly common for a 20 year old radiator to be partially clogged with rust and casting sand. A good radiator shop can rod one out. While your at it, you can replace the thermostat. If the thermostat is sticking open the coolant won't stay in the radiator long enough to cool back down.
Is it loosing coolant? If it's not, that pretty much rules out an expensive engine repair, like a blown head gasket or cracked head.