Have this problem with an Ecoflam Minor 1/WM Grandee. Burner goes to lockout when trying to re ignite after reaching temperature on the boiler stat. The flame will re establish for approx 10 seconds. After trying for about 6 lockouts will fire & run. New solenoid coil, motor & photocell fitted. Co2 10.5%. Pump pressure has been pushed up to 140psi to enable clean burn. There are some issues with the height of boiler in relation to the oil tank about 1m negative head.