oil boiler seldom lockout but especially after first 10 mins?

a Trianco TRO boiler with a Monoflame oil burner. It fires first time and can run all day but it will also fire first time, run 10 mins then lock-out. Oil pressure fine. Solonoid on Delta pump is brand new. Doesnt sound like a photo-cell prob. As its in a cellar its difficult to always be with it when it locks-out. Control box with intermittant fault or maybe HT transformer ?? Any ideas please