Lower the setting and test water temperature. If water temperature does not reduce at a lower setting then the thermostat should be calibrated. Raise the setting and test water temperature. To decalibrate thermostat Flow through the heater is too high. Turn down the volume of water flow to stay within the specifications of the unit.
Gas pressure is too low, because gas line is undersized or constricted.
Check gas line size in the specification table to ensure it is adequate. Gas line size table Check gas pressure at inlet tap on unit to ensure it is adequate. If water is mineral ladden and unit is fairly old the heat exchanger may be scaled internally. This prevents efficient heat transfer. Removing thermostat probe from left side of heat exchanger will allow you to see some of the heat exchanger tubing.