The problem could be a slight change in the initial boiling point of 100LL.  The specs for 100LL may allow refiners to vary just enough to cause fuel to start to bubble in the lines at a temperature and density altitude that was OK previously.  The boost pump would solve this problem most of the time with 100LL; with auto fuel, maybe not.

About leaks.  Fuel has lower surface tension than air so it will leak when air won’t.  I learned this when air trapped in a fine screen Gascolator with a glass bowl in an Aircoupe.  Fuel would pass thru the screen but the air bubble in the top would not!

Steve   Legacy

At 6,500’ the engine began surging.  When we turned the auxiliary fuel pump on the engine immediately ran fine.  We have now had the engine driven pump overhauled.  We have pulled vacuum through the entire fuel system and we have no leaks.  We replaced 100% of the flexible fuel lines.  Metered and un-metered fuel pressures are reset and fine.  We are at a loss as to what is causing our engine surging problem.

Berni