I leave my AC on year around. I'd check to make sure you're getting good air flow through the AC condenser. Make sure it's not caked with dirt. Also, moving to a lower viscosity fan clutch (blue fan clutch) made a big difference for me. The fan "roars" now where it didn't before. If I pop the hood open while the engine is hot and running, it's a windy place to be.
In all, I think it's a combination of many things that are all attributed to age. Flush, new radiator, new thermostat, new water pump, new hoses, new fan clutch. All together, it runs like a factor fresh cooling system - because it basically is.