For the last 2 years the gas combi boiler we have has not been consistent delivering hot water. The CH appears to work fine. When hot water is demanded the water comes out hot but after a minute or so will turn cold. It appears the bolier stops heating the water (even though the tap is still running) then it will start again (you hear it fire up again). It makes it very hard to have a decent shower as its hot one minute and cold the next. Pressure in our area is good and even when I only run the hot it shows the same problem. Can anyone offer advice as we have recently had a registered CORGI engineer around to service the unit but it didn't resolve the problem. Thanks in advance.