After shower water temperature is reduced to a comforatable mix, it will in a short period goe to cold. The single handle Kohler valve has been replaced. The shower hose has a flow restrictor. The water is heated by a Noritz tankless. Temperature control is not a problem at other locations in the house.

I need help troubleshooting this problem.

So you are saying that this problem existed before and after the valve was replaced, or did I miss read you? If you open the hot and cold sink faucet will the temperature remain constant for a long period?

Problem existed before and after valve replacement. Temperature remains constant with the hot and cold sink faucet. Lowering the temperature from 120 to 110 did not solve the problem unless you run the shower on hot only.

I am guessing the flow restricter in the shower hose is causing the flow through the water heater to run below the minimum. However the problem did not start when the shower hose was first installed.

I am still puzzled.

The only time I have run into this type of problem was with a real cheap valve and Kohler is a pretty good brand. I too am puzzeled. I would get to Kohlers consumer help desk and ask them for advice, there may be a defect or fault that they can cure.