Opie, how did you measure the 260 deg temp? I installed a separate sensor next to the original one (there is a tapped and plugged hole next to it) and found that the dash gauge isn't very accurate. Mine is off by several degrees.
Also, when you take the old one out or get a new one, put it in a pot of water on the stove along with a thermometer and observe it open and close as you heat up (and cool) the water.
Finally, as a routine maintenance check, check the coolant with "dip strip" for acidity, and freezepoint at least. It should be clear and light green (ethylene glycol). You probably need to change it , if you haven't already.