Alpha 280p - Intermittent hot water and heating

[ajf01923]

I have an Alhpa 280p which has begun behaving intermittently. When turned on the hot water will be fine for a minute or so then the heater will cut out and revert to cold. If i leave the tap on the heater will then cut in again - hey presto hot water again. The same applied to the heating - burner will cut in and out of own accord. Any help or advice would be greatly appreciated. Andy