On Sunday, February 17, 2019 at 4:07:32 AM UTC-6, Alan Holbrook wrote:
The water pressure in my domestic hot water supply has recently dropped to
very low levels. The water temperature is fine and there is no problem
with the cold water supply. When I open a hot water tap anywhere in the
house, the water dribbles out for a while and then suddenly gushes for a
couple of seconds and then reverts to the dribble. I have an oil fired
furnace and a tankless water heater...

Unless you have very "soft" water a tankless water heater requires
regular descaling to keep them from clogging up.