Audi A8L 4.2 doesn't start. Air bubbles in fuel

[iGO]

Hi
One day I started my car, but after few minutes its start dying, revs was slowly droping down and acceleration pedal didn't respond. After car stalled and didn't start any more.
Car already over one month in garage and they can't found what wrong. Just what they told me, that fuel coming from tank to engine with air bubbles. Fuel pump is working.
Maybe someone had same issue or knows where to check?
Because its looks like they will never fix it.

Thanks for any help
Igor