2001 Subaru Legacy Flucuations in Engine temp w/ AC on

On very hot days, my Legacy engine temp often runs very hot while running the AC, particularly during stop and go driving. On the highway, I notice the temp rise on long hills (when the RPMs are at ther highest). When the engine temp gets high, I turn off the AC and the engine goes back to a steady-state temp. I checked the two radiator fans and found them both running (though I'm not sure they are running at peak efficiency). Coolant in the radiator is full and relatively fresh. Is something (relays/module, fan motors) wearing out or has a specific component failed?