You may have a partially-cloged radiator, but keep in mind...

Water in a non-confined space (radiator with cap off) boils at 212 deg F. The purpsoe of the pressure cap is to increase the temperature at which water boils. It raises the boiling temperature about 2-3 degrees for each lb of pressure. So, a radiator with a working 15-lb presure cap won't boil until the water temp hits 240 degrees or more.

How does your coolant loook? Is it nice and clear green (or whatever the colorof your antifreeze)? Oris it brown and rusty looking>

Also, remember that you can sometimes get rid of a lot of gunk in the cooling system by draining out a pint or so, pouring in a can of cooling system cleaner, and then subsequently back-flusing and cleaning the radiator, and filling with new coolant mixture. I'd try that before spending the money on a new radiator.