Constantly have to clear an airlock in my Riello Burner?

I have 50 gallons of oil in my Tank. to start the boiler , I must bleed the valve and purge out the air, thereafter the burner fires. Once it reaches the high limit setting it cuts off(as it should). When it goes below the low limit setting, it fires up and only stay on for approximately 5-7 seconds. In order to star the burner, i repeat the same procedure in purging out the air until i see a constant flow of oil coming out of the purge valve and thereafter I close it and the burner fires up again. This is the manual process i do in order for the boiler/burner to work. It seems I constantly get an air lock after the burner shuts down. I then have to repurge the air lock in order to start the burner/boiler. Also. I want to add by pressing the reset, the burner starts but does NOT stay on(because of the air lockout).What do you think is wrong?