Thermostatic mixing valve that holds water temperature at 120°F / 50°C
A thermostatic mixing valve is the same device as a thermostatic shower, the mechanism is the same but the body &amp; fittings different.

So, have you tried adjusting the existing valve? Is there any chance it has got some debris in it causing it to fluctuate. Is the cold supply to the mixing valve restricted?

It may be a faulty valve which needs replacement.