You either have:
A) A leaky radiator
B) A leaky radiator hose
C) A bad water pump
D) A frieze plug popped out.
E) A blown head gasket.
If it is the radiator, you can try to plug the leak, but you are better off just replacing the radiator--sometimes they get clogged up and don't work like they should--that sounds like what might be happening.
If it is one of the hoses, replace it.
If it is the water pump, replace it.
Frieze plugs are little slugs of metal that are used to plug the holes where they drain the sand out of the block (the sand creates the voids in the casting that become the water jacket, etc) sometimes these will leak or pop out when it gets hot--they make replacements that are made of rubber with a screw on them that makes them expand out. (There is one behind the motor mount on at least one side--it'll make you think you cracked the block until you take the motor mount off).
If your head gasket is blown, then you will need to replace it, and hope that your heads and engine are still good. This will cause it to blow steam out one (or both) tailpipes, plus it will lose power.
You might also check your thermostat--they can get stuck. Flush the cooling system really well, and put a new thermostat in and see if it still happens. If not, it's probably one of the other things.
I'm just about to replace my belt-driven fan with an electric one. I'm not going to get fancy--just grab one from a junk yard and wire it to a switch so I can flip it on when it's sitting (my '65 mustang usually doesn't even get that hot when its cruising down the road).