OK, I have researched this and I am still stumped. I have a 2001 LS V8 and I am getting a slow heat creep. The car will drive fine and the temperature will rise accordingly and then stabilize at normal operating temp. After about 20-30 minutes the temp will begin to rise to about the 2/3rd mark but not above. I replaced the thermostat about 2 months ago and replaced the hydraulic fan with a 3200 CFM electric with a shroud at the same time. The degas bottle releases pressure when I open it and I have purged the system several times to make sure it is not an air bubble. The outside temp is 75 degrees and I am not running the A/C. Turning on the heat to 90 degrees does not make a difference. My upper radiator hose blew a little over a week ago. I replaced it and did all the bleeding, etc. and all was fine for 4 days before the heat creep started. Any advice would be appreciated. I am wondering if the water pump is beginning to go.