how to resolve this problemwith rdb burner?

burner is 3yrs old i did a full service also combustion test results were fine the burner will work fine for hours then out of the blu go to lockout press reset and work fine again for perhaps all night next morning gone to reset tried new all electrictial parts control box; coil ,photocell ect could it be oil pump idid test pressure it was ok any ideas would be very apreciated yours jc