Bubbles not horribly uncommon when pressure drops in a fuel line. Subsequent compression by pump may not be enough to collapse the bubbles. A very small leak can occur at a barb fitting resulting in a "valve" effect drawing in air but not letting fuel out (not very common, usually you get a messy leak!).

SU's can handle a lot of air in the line but get enough air and your top end will suffer.

Mike!