thinking out loud for a moment...

you mention that you've got a separate guage, how do you have it patched in? is it possible you have a bad sender?

it's odd that you'd be running THAT hot and not at least spit SOME coolant out the overflow...

i'd think that if the thermostat was stuck, the temperature wouldn't level off, it would just keep going up until it eventually "blew it's stack" and vomited up all the coolant...