Lets try to find what its not. Try to eliminate the obvious.
Maybe something is sticking. If its a switch or valve (smart valve maybe ) that is sticking, try tapping everything you can to possibly get it unstuck. Things like this happen when furnaces sit for periods of time unused.
To be sure the regulator is putting out enough pressure. ( a quick n easy field test)
1. Let the water heater cool down .
2. Turn on the stove burner then turn the water heater on while you are watching the stoves blue flame .
the flame on the stove should remain the same when the water heater fires up, no fluctuation.
sometimes, the regulator diaphragm can stick or even crack in very cold temps .
I wrap my regulator in a little insulation.