I think I'd install an indirect water heater tank. It sounds like you have an in boiler coil. It might need demineralization, or the aquastat is not keeping the imersion tank hot enough, or it wasn't installed with the proper flow restriction so it can pick up enough heat. If it has a tempering valve, that could have failed, and it is mixing too much cold water in. Those are adjustable, and if you have one, I'd try that first. It would be on the output of the boiler, with both hot and cold inputs, and the mixed outlet goes to the rest of the hot water users. There may be a knob or a hex key hole to enable adjustment.
If the boiler has a temperature readout, what temp is it set to or holding?