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Water gets hot, then cold


I have a boiler with a steam system, and no hot water heater in my house. The past few days when I go to take a shower the water will get hot for a minute, then run lukewarm. If I turn up the temp it will get hot again and then cool off again. It only seems to give me hot water when it's set to full hot, of course then it's scorching.

This same issue seems to be happening in both my sinks as well, so I am pretty sure it is not the shower.

I was told a while back by the guy who serviced my boiler that those signs mean the mixing valve is dying. I had a friend replace the mixing valve coming off the boiler today, and at first it seemed like it was fixed, now it is back to the same issue again.

Is there anything I can do to check and be sure where the issue is exactly? What other things can cause this issue?
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