Engine overheating

This vehicle began overheating when the AC is on after about 30 minutes. The overheating has literally blown the cap on the radiator and spewed coolant everywhere twice. Ford thought it was the thermostat and replaced it 4 times, thought it might be a faulty switch giving a wrong reading but obviously not since when it reads in the red it does blow. We had the water pump replaced thinking it was that but $500 later same problem. We noticed it overheats too when their is "stress" on the engine, like going up long hills or inclines (we live in Colorado). So the engine overheats with AC and stress on the engine. Any thoughts? It has low mileage, nice car but the threat of overheating takes the fun out of driving it. It seems to be impossible to fix this. Thanks!