Camray 5 external boiler will not run continually?

Boiler works fine for HW and CH for a few hours then keeps locking out. have to press red button on burner. will fire for a few seconds then locks out again. (sometimes then runs OK other times keep having to restart again and again) Oil level is fine, have bleed oil feed and no air lock. all dials etc set correctly. Could it be that selanoid? is faulty???? Please help as cant get anyone to come out over xmas and new year period!!