Dale,

Just doing a little research and few have reported similar symptoms due to air in the line. They opened the bleeder valve on top of that polygon shaped housing and all is well.

I'm not sure which sensor controls the shutdown. I would have thought one of the many hanging from the thermostat housing. However, someone reports it's on the top back side of the head. I'm not sure where the back side is in the reference. http://powertech.myshopify.com/products/05stkub03