I think the first thing to do is to check the oil pump pressure - normally they run between 110 and 150 psi. The burner will only run as expected if the combustion areas and flue are clean, and the burner nozzle is recent. Your system is probably cutting in and out with regular frequency....it automatically re-ignites if it blows out, but only within a specific time period. Maybe the thing just needs a service?
John
July 2008