I think that you should check all the things mentioned. If I was you, I will also check the radiator cap. I had a similar problem once. My water was getting out of the reservor. It seem that is was boling but it was not. My raditor cap was so bad that the spring inside of it did not work. The spirng allow pressure to scape of your systmen while you drive. Since the preassure could not scapebecause the spring the not allowed teh cap to move up all the preassure went into the reservor forcing the water to come out of the reservor. By that time I had changed the thermostat but I noticed it was just the $8.00 cap. The only think that was differnt form your scenerio is that my gage always gave me the overheating reading.

Jorge