We recently added onto a 1920's bungalow, adding a master bath and tankless water heater. When we take a shower in the new addition, the water temperature fluctuates wildly from a bit too hot to too cold, back and forth continuously, there is no middle ground. At first we thought it may be that the tankless water heater wasn't keeping up with the demand on the warmer water. However, when we go take a shower in the original bathroom (fixtures remodeled about five years ago), the temperature in the shower is consistent for as long as we want to shower. The new shower is all over the map within minutes. We have a Kohler fixture in the new shower. What can we do to diagnose if it is indeed a fixture problem (as it is the only place in the house this occurs) and what we need to do to get it fixed?

Thanks!