air bubbles in fuel rail/tank?

hi- i have a renault trafic 1.9 '05 plate. issue started instantly- just cut out as i was driving. half a tank of fuel on guage but AA guy said there was air bubbles running through the priming pump rail... got it started a few times but kept cutting out til eventually priming wouldn't pull any fuel through. had mechanic change new priming bulb/pump. seemed ok but within 5 miles going uphill it conked out. got it restarted and got to nearest petrol station. filled it up with diesel and hey presto no problems.. until a few days later when guage read 3/4 full driving downhill it conked out again. diff AA guy came out said it was an issue in the tank as when he emptied a can of diesel in it-it started!we refuelled it to the brim & no problem. now mechanic has dropped the tank and found nothing unusual. any clues as to what's causing the air in the fuel and why i need to be more than 3/4 full for van to run ok??? thanks