The only other thing that springs to mind is the water pump, I don't know about your particular car but some pumps suffer with the impeller coming loose on the shaft. Does your heater work OK? That's usually a good indicator as to whether the coolant is circulating properly. They has been a spate of faulty new thermostats lately, if you still have the old one try boiling it in a pan of water and see if it opens, likewise with the new one.

hi, it looks like problem with temp sensor or with wiring system that i can check as well. ;)

Hi, Did you find out what was the problem ?, I am having the same problem, get the car out in the morning drive a 1/4 of a mile then the temp engine warning keeps buzzing, once its showing one pip on the temp gauge everything is ok, another thing if I leave it parked on a hot day for a couple of hours then start the car the fan come on right away, I am going to try a new thermostat and sensor. Thanks Steve