Stickandrudderman
Probably a blocked/inopperative venting system. Air may not be able to enter the tank to replace the fuel being used.
There was once a DC4 at Perth, Oz, which had been topped up with cold fuel. Standing in the sun fuel began to vent from an integral wing tank as the fuel expanded. Someone plugged the vent. Soon after take off the wing skin buckled inwards as the tank booster pump worked hard to keep up a flow. Could have been catastrophic. Murphy just loves to stay in the limelight!