98 Chevy Tahoe 4WD 5.7 350 My truck was running cold, was not generating heat, I pulled the thermostat to find that it was stuck open. I replaced it with a new one, filled up my coolant and went for a drive. It worked perfect for the first 20mins, I had heat and it was regulating at about 200 degrees. Then all of a sudden my temp gauge started fluctuating, getting really hot then cooling down, it stayed mostly around 260, it was over heating pretty bad. I popped the hood and saw that coolant was leaking around the main hose that connects to the thermostat housing, the hose had so much pressure built up that I could barely squeeze it with my hand. There was no hole in the hose, it just had so much pressure built up that it was pushing past the hose clamp. That lead me to believe that the brand new thermostat failed and was stuck closed. So I went and bought a new one, replaced it and it did the same exact thing.....worked great for about 20mins then failed. I also checked the 1st new thermostat I bought by boiling it in water and it seemed to be working fine. Im not sure what is going on. What else would cause a thermostat to not regulate properly after 20mins of driving?? The water pump and radiator were both recently replaced about 6 months ago. There is no leakage on the inside which leads me to believe that the heater core is ok. Ive also let it run with the cap off to make sure there was no air lock. Im stumped.