All most all of the problems I ahve dealt with recently seem to involve a blockage in the raw water system. That is easy to check. Do you have plenty of water exiting with the exhaust? Are you getting steam coming out with the exhaust? Are the "cheeks" on the heat exchanger cool to the touch? Answers shoud be yes, no, yes.
I had to deal with a blockage right at the nipple in the exhaust manifold where the raw water exits before going into the mixing elbow. A friend had a blockage right in the exit from the raw water pump.
I will be back home tomorrow and post some pictures of the thermostat location.
John Brecher