combination boiler and hot water heater issue

[Joe Xie]

I have a unit with a Baxi's Luna 310fi combination boiler and hot water heater, tenant report that hit water cut off often during shower. "The shower goes cold, not gradually but suddenly for about 3 minutes in the middle of the shower about 3 times a week. I take on average 10 minute showers, I would estimate that it happens a couple of minutes into the shower. There does not seem to be any correlation to anything - something else running, previous use, amount of time of use before- nothing. There is no warning, it happens and then gradually reheats." What could be the causes? How much it could cost? Thanks in advance for you inputs.