hey need a bit of help my ford el its building bulk pressure in fuel tank enough pressure to make my el not run properly, if you unscrew the fuel cap it release all the pressure obviously and the it will run properly again, as soon as you screw the cap back in the car will die, would this be just a new fuel cap or is there a breather somewhere that is blocked. so if someone could please shed some light on this problem would be very much appreciated. cheers

check the canister purge solenoid and the line to the tank.. this is the breather for the tank.
the solenoid is in the engine bay next to your air filter box and the line runs from your manifold to solenoid then from solenoid to carbon canister then from canister to the filler neck on the fuel tank..