2000 Chevrolet Silverado Silverado 5.3 vortec overheating issues

I have a 2000 silverado with a 5.3 vortec engine. It has been overheating on me for the past month or so. It started out as only overheating on long hills and then would cool back down to normal after I made it to the top but it has progressed to where it will now overheat at normal highway speeds of after idling for long periods of time. It seems to cool it down when I turn on the heater but sometimes this doesn't work. I have flushed the coolant although I didn't use the prestone flush and I replaced the thermostat. The big thing that gets me though is that if I pop the hood even after the guage is showing 230-240 degrees I can still touch the top and bottom radiator hoses and they don't feel hotter than normal and there is no indication of it overheating. I haven't had to add any coolant and it is circulating. Any help would be greatly appreciated!