oil fired burner problem?

hi i have a eurocal boiler with an external sterling burner,i have this serviced anually (cleaned by a qualified heating engineer).however it still seems to have the same old intermittant problem.where when its turned on and fired up it cuts out after anything from 1 minute to four hours.then it needs to be reset red button and same thing happens again.the heating engineer cant trace the fault no matter how many times he checks,and says that the only way to trace the fault is to start buying new parts one by one ,which is a road i dont want to go down.reading this forum it could be the solenoid?is this true?.
many thanks.