euro cal boiler oil fired not combi?

i have euro cal boiler approx 8 years old ,the problemis
this the boiler will fire up and run for an hour or somes days 2 or 3 hours no problem ,however on other days it will run for an hour get to temp then stop and the burner will try to fire up but fail , you can here the fan run and then it fires to fire just before it fails you can hear the boiler stat click like it says the bouiler is up to temp then if you wait a hour or so till it cool downs it will fire up , what do you think it is