First, you need to decide what kind of water heater you have installed - a flow-through or a storage one. The reasons why the water is sometimes cold and sometimes hot are fundamentally different for such water heaters. Here you can find more valuable information about the gas hot water system. The search for a defect should begin with checking the pressure at the inlet to the storage water heater. If it is normal, then we check the pressure at the outlet. If everything is normal there, it is necessary to seriously check the water supply system itself.