Intermitant air in fuel

I posted a question a few days back titled "Air in fuel" i have replaced the fuel filter it is a sealed unit, there is air in the return pipe occasionaly when the engine is reved i am ashured this is normal by my renault deler ship, the car has run down to near empty and the car has not conked out which it did last time, I keep a carefull eye on themilage and it usaul does 480 to a tank full sometimes air appears in the fuel feed line ( direct from tank) at around 400 this is when it conks out. Bit of a mistery. Have tried some fuel cleaner, there is no water in fuel( have blead filter) no apparent leakges from around the small pipes flowing away from the injectors, I may try cleaning out the tank, I don't know if this will help??? Any ideas, thank you for the two answers I recieved on my last posting.