hummer h3 thermostat

Does not have to be a regularly serviced item. Most often I replace them due to a check engine light and P0128, fails to reach operating temperature, code. I changed one in a 2008 last Monday that was improperly regulating temperature, that had no code. The customer complaint was that the gauge moved around on long drives. I determined with a scan tool, that the engine would take a long time to reach about 175, and picking up the speed(increasing flow) would actually drop the temperature about 10 degrees.