I had nearly the identical symptoms. In my case, it was a bad ground causing the gauge to read hot(ter than normal). Anyhow, the temp sensor is pretty much indestructible, the thermostat is very easy to test, and new pumps generally work pretty well...so I would think either the gauge is done or the aforementioned grounding problem. Check you wiring diagram. Or you can follow this general rule of thumb, "if it's hard to trace and annoying, it must be electrical!"

L