Hello. What size gas line is feeding the water heater? Or I should say, what size gas line is coming out of the wall? 1/2" or 3/4"?
These heaters are all the same in the respect that they need a minimum gpm(gallon per minute) flow to make the heater work correctly. In other words if your shower head is using 2.5gpm, then your inlet flow has to be at least that. If there is no filter on the inlet cold side, you definentally need to install one. Tankless heaters are very tempermental if not kept clean internally. Do you have the manual for this water heater still? If so look up the flushing procedures. This will clean all of the internal piping within the heater. This should fix the problem.