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Bought a 2006 Sierra crew cab w/5.3 a few weeks ago. One day it got up to around 230 degrees, I turned A/C off and it cooled back down. it was only 85 degrees outside. I also noticed it took a longer than normal time to get up to temp, so I bought a thermostat for it. Before I tried to replace thermostat, I added Peak Radiator Flush to the radiator and drove it a couple of days with no problems. When I removed the breather, to replace thermostat, I noticed it had a new water pump. It also did not have a thermostat when I opened her up. It did not have antifreeze, only water. I installed thermostat and filled with water - drove it a couple of days to work (approx. 5 miles round trip), and the gauge read exactly as my 2004 Suburban always has, just under 210. When I saw there were no leaks, and it was performing correctly, I drained water and added Dexcool and distilled water 50/50. It did fine for several trips to work until yesterday when i drove it to lunch, it did it again. I tuned heater on high and it started cooling back down. Drove home that evening with no other problems. Any ideas? Thanks