You may have a problem inside the heater that is causing the heater to pop the reset button out when something is wrong.

The limit switch is shutting off and you are having to press the re-set button, right? The limit switch, if I am understanding you correctly, is the thermal disc type, may be ok, but what is causing it to shut off? I would first, not put total faith in the serviceman (PG&E maybe) and replace the limit switch. Its only a few bucks,and easy to do. Then if its still the same problem, look into the venting pipe. As it may not be adequate for the heater. Espically if its new. These new wall furnaces are kind of touchy. It may be larger, BTU wise, and the vent pipe may need to be enlarged. If this is a big job, and a bit too impractical, check the vent pipe, make sure its in good conditon, no leaks ect. install a cO2 detector and by-pass the limit switch.

Just ensure the connections are proper .

Also ensure the board is getting proper power supply .