Mike, I bet the thermostat got debris in them. If they haven't been replaced in awhile, I'd do it. You can put them in a saucepan with hot water to get them to open and get some tongs and move them back and fourth. Most of the time, you'll see wood chips. If you decide to reuse them after you clean them and you see them cycle at the right temp, check the rubber seal and replace gaskets (if not perfect). I replaced mine, just because, a year ago and I think they went to a different part number. I think it was a little higher operating temp... I can't remember 100%. I too have the XB.
Contact Rockwall Marine or Don Weed on BBC for clarification and/or parts.