The one thing that comes to mind that has happened to me after all the obvious has been looked at and/or replaced...... is the high speed needle seal. Exact same symptoms you are describing. It never leaked fuel but was just "loose" enough to cause a problem only in vertical maneuvers. Level flight and mild angle climbs up to 45 degrees... no problems.

I am assuming this problem your are having is does not change regardless of the amount fuel is in your tank.