I have 2 ideas, the first, alot of people are gonna completly disagree with me, but this is just a test, not a fix. TEST
Take out the thermostat. Drive without it, if that fixes the problem, then you may need a new thermostat. You may be like me, they way that you goto advanced auto parts or such places to buy most things? If so, I've see people buy a thermostat from Autozone, and they crap out within a week. Sometimes the quality is crap, sometimes its good, its the luck of the draw.
Are you sure your gauge is right? I had a wiring problem one time on my ZX, that it would say I was overheating when the motor was just at operating temperature. At times it would even "overheat" by the time I got out of my nieghborhood, about 1/2 mile.