Oil fired boiler problem?

Hi can you assist,
Grant Multi-pass 70/90 oil fired boiler, has had two new photocells just to confirm first one was not faulty, a new control box and a new nozzle of the correct size, confirmed by Grant UK. have checked oil line filters and pump filter, motor runs and pumps oil in steady stream, fan appears to work fine, solenoid on pump works, tries to fire up and keeps trying sometimes goes to lockout sometimes just keeps trying to light? do I replace the oil pump? not sure what else to do have also tried changing air damper settings and pressure settings slightly but makes no difference.