Boiler firing problems?

I am having trouble getting my boiler to stay fired when I press the on on. Sometimes it fires fine and will work for the full hour.Other times will stay on for say 5mins then other times it clicks on then cuts off immediatley. The piolot light is staying ignited so I know it is not that.And the programme box seems to be working ok as I have just programmed it again. I have checked the connections and all seem secure. Any suggestions please? Also it is a Glowworm Economy Plus EP40B - old type boiler