I agree on the probable dirty/ worn nozzle. How is the light-off after you reset the flame safety control? How does the burner sound during a run cycle? Is there by chance a solenoid valve on the nozzle supply line between the oilrack and pump. Pump pressure, oil quality/ volume could be a factor. The 'eye' and 'oilrack assy.' are probably also dirty.
There is a good chance a nozzle and good cleaning will get you through to the 'changeout'. Best of luck--