How can I fix Glow-worm Micron 60FF?

I have this for more than 3 years and it has been working fine with CH and HW, until a couple of months ago.

Every now and then, about 3 to 4 times a week this was what happened:

1. The timer is programmed to turned on the CH and HW at 15:00.

2. At about 15:45, when the HW and CH reached the pre-set temp. the boiler was turned off.

3. At about 16:00, when the boiler is need again, the timer switched on the boiler

4. The boiler fan was on and the ignition sequence kicked in place.

5. However, the burner was not on after a few clicks and the fan just keep going.

6. If this happened, which as I said only happened 3 or 4 times a week, I reset the boiler, either by turning the thermostat to min and then to the normal setting again; or just by switching the main off. The ignition sequence kicked in again and the bolier lighted up and there will be no more problem for the rest of the night.

Please help.