The first thing I'd suspect is the t-stat. Then the radiator, followed by the water pump. Most pumps crap out by blowing the bearing and leaking before the lose pumping capacity, but it could happen. Other than that, there isn't much load on the engine.
You can't really tell how hard the a/c is working because the condenser is ahead of the radiator. That's why the watem temp goes up when you run the a/c, you have a freon to air to water heat exchanger.