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?