Car stalls with A/C at full tank and without A/C at half tank
 

Hi. My 2005 Ford focus hatch back car has fuel transmission issues. When the fuel is gauged at full tank, the car runs perfectly for about an hour with the A/C turned on and then stalls thereafter even while in motion. When I turn the ignition off and on, it works again for 10mins and the same schedule is repeated. Without the A/C, the problem occurs at half tank. The tank stock doesn't go below half else it wouldn't run. I have tried 4 different pumps and changed the filters. I have changed the fuel relay in the engine compartment and the power supply cable. Diagnosis doesn't read any fault. What do you suggest could be the problem?