My truck is a 1999 silverado z71 w/ the 5.3. its got almost 200k miles on it and has always been reliable. Last week I was hualing a jeep home on a trailer and it got hot while driving in 100 degree heat w the AC on. I pulled over "popped off the cap" literally and had to refill w water. limped back home slowly. The next day I had the radiator flushed-$62. Drove it to work and it was good that morn but got hot again (235 degrees) that afternoon on way home.i had to turn off AC and turn on heater to cool it down to 210. I replaced the 15 psi overflow cap, thermostat, waterpump, and lower hose- same thing. Today i reflushed the system myself at home by removing thermostat, and replacing lower hose onto waterpump- running a hose into top of radiator and turning on heater to ensure the heatr core is also cleaned out and flushed entire sytem for 30 min while cranking the truck sporadically and it still gets hot w the ac on. I never remember it getting hot like this before last week? I don't know what else to do- I can drive it w no AC on and if it gets warm I can turn on heat to keep it around 210 but I'd like some ac in this heat.