Has anyone else had this problem? My 05 V6 keeps doing this. The temp gauge will just shoot up past the H and the motor is at normal temp. I've taken it to the dealer twice. The first time they replaced the thermostat, then a sensor. Now, they ordered the whole cluster for me.

 

Okay so they ended up replacing the whole gauge cluster and that fixed it.

 

Do you notice any moisture on the passenger side carpet near the kick plate? There have been so many S197 owners plagued with the dreaded water leak. A water leak will fry your electrical circuits and cause things to go hay-wire. Reports from members of other Mustang forums, have found some of the causes and can be easily fixed.
So far (knock on wood) I haven't had a problem.