riello 40 gxb burner ??

i have the above burner on a fire bird combi -about 8 years old ,the system ran out of oil and sucked cr*p in ,which has now been cleared ,from tank pipe and filter on pump , a new nozzle has been fitted .got the boiler running ,it ran for about an hour then locked out,i stripped it down again this time removing the brass valve, cleaning it and the boiler fired up again ,for about an hour, could it be the o rings on the valve? or am i barking up the wrong tree ,i would appreciate any advice