Tiger loop empties sporadically?

This is an odd one - The reservoir on the tiger loop and oil line empties but not all the time - tried a new tiger loop and that was worse, may be a faulty one? all joints have been remade, oil line pressure tested,even installed a non-return valve and that made it worse? also tried new oil line 8mm rather than 10mm which was originally put in. Installation is only 4 months old, everything brand new - Grant external vortex 15/26 boiler - grant engineer installed 2 new oil pumps, it wasn't that! - Oil line length is about 23m lifting just over 1m. Indications would infer that air is getting in somewhere but where and how? - any ideas? Cheers Dave G