A Tigerloop should be able to pull up oil to 3.5Mtrs, so unless your boiler is in an upstairs room an oil lifter should not be needed, I am concerned that an Oil lifter has been fitted externally as these are Electrically operated and generally are not weather proof???
Sounds to me like you may have had air leaking into the suction pipe to your Tiggerloop, or there is a problem with the oil burner such as oil pumnp of solenoid coil etc etc
www.elanar.co.uk, November 2009