Halstead Ace high CH not right!?

20 mins after putting heating on, "power on" light flashes, heating light stays on but burner light goes off. When i turn on hot water, HW light comes on but heating light goes off, power on light still flashing. I turn HW off and all lights go off except power on light which is still flashing.
After hearing a "gurgling" sound from the boiler, i switch heating off and 2 mins later, the power on light stops flashing. Hot water goes back to normal.
Anyone know what's wrong?