Hi guys. Long time lurker, first time poster here. This site is great! I have found other similar threads to this but they have either gone unsolved, or they were fixed by something I have already tried. I'm looking for help on where to go from here. Here is the story: This spring, I replaced the water pump on my 04' Trailblazer 4.2L due to worn bearings. Part # Gates 41122. I also "flushed" the radiator with a DIY fluid from Prestone. Then filled with 50/50 distilled water and DEXCOOL. A couple of months later I had the first overheat while pulling my boat on a hot summer day on the highway. Coolant was bubbling in the reservoir. I had the car towed home and I replaced the thermostat. Part # AC Delco 15-11006. The new and old thermostat were tested with boiling water and both seemed to operate properly, which made me think that was not my actual problem. I also replaced the radiator cap, but did not use AC Delco brand. Filled again with 50/50 DEXCOOL. The truck seemed to run fine, but would still occasionally overheat when pulling the boat or running the A/C on a hot day. I lived with the problem until about a month ago when the truck overheated badly and unexpectedly on a long highway drive at 70mph not towing anything on an 85 degree day. Engine had to be shut off to prevent damage and towed home again. Coolant had been pushed out of the radiator and into the reservoir, creating loud bubbling sounds again after the engine was shut off. I filled up the radiator again and drove it a few days short distances without a problem. Then the vehicle started overheating slightly almost every day driving around the city. So the last part I tried was a genuine AC Delco radiator cap AC 85. I am still having the same problem. If I fill up the radiator while the engine is cold, the car will not overheat for 2-3 days until it has time for the coolant to be pushed out of the radiator into the reservoir. There is no coolant smell in the exhaust. No visible coolant in oil. I have used a block tester with fluid and no exhaust gasses were detected. The fan clutch is engaging, which I have verified by sound, (seeing overheated dash gauge = jet engine sound) and by hand (trying to stop the fan at idle while the car is overheating. I also tried my best to get the air out of the system with the rad cap off. To recap things that were replaced this year (in order): Water Pump - Gates 41122 Thermostat - AC Delco 15-11006 Radiator Cap - AC Delco Any other ideas guys? My only idea right now is to have a radiator shop look at the radiator. I am not sure if/how they can test for blockages. Thank you!!!