biasi combi boiler, hot water problems

The hot water is sometimes fine then it runs cold, I've had the diaphram changed I originally thought the water pressure was too low that was causing the problems, I've had that checked by the water company but it's fine, also the water pressure in the central heating drops below 1 bar, I top it up to just over 1 but then over night the pressure seems to rise to bewteen 3 & 4 but at the moment the hot water is working ok the boiler is about 12 years old and has been serviced regually