you can try all the normal stuff like
fluid thermostat & sensor.

i would do sensor last.
I experienced a similar problem with my buddys stratus a couple months ago.
Just like that the gauge would shoot up for no reason at all. We did all the regular stuff....flush thermo and even sensor. same thing. The fan worked even with ac/fans on...it would shoot up at the weirdest moments.
Turned out he had a *gas* leak from *i think* the combustion chamber.
if you try everything and still no go let me know and I can check with him to find out exactly what it was.

BTW...we took the stratus to this guy who used a chemical to diagnose the problem...he captured some fluid and put in a chemical and looked for a reaction.