Where did you get the shower head from? I have exactly the same issue and have already put in a replacement head to no avail. The Towngas guy just took the shower head off and said look, it's working fine now. You just need a better flowing shower head. Don't want to keep buying shower heads for trial and error. Should I just go to the Towngas shop and pick one up or is experimentation required?
How do you control the gas flow? The panel just allows you to adjust the temperature while the heater itself seems to not have any openings, so not exactly sure where the gas flow valve is located (assuming it's on the pipe leading to the heater).