boiler cutting out, hot water goes cold?

I have an Alpha Ocean 240 eco boiler and it keeps cutting out. The hot water heats up but then goes cold. It eventually warms up again but then goes cold again. Help! British Gas have looked at it several times to no avail. They have replaced the diverter valve and something to do with the air pressure but the problem persists. They are talking about replacing the fan now. Will that help?