you are probably going to have trouble with
that faucet because the tankless water heater has
sort of lowered the pressure or the volume on
the hot side of your system........
those tankless units can be a pain in the ///
I am sure you will get some advice form others here
I have heard of installing a thermal expansion tank on the
hot side of the system to keep the volume up
also changeing out the faucet to a non pressure balanced one
might also be a way to resolve the problem.
good luck --have fun