It starts pretty easily but wont run longer than 5 sec? Replaced autostop relay.

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Its had the fuel pump changed/filter, throttlebody replaced, swapped distributors, it will start and even rev way up but only a few seconds then its like you turn off the key... every single time.. please help!! Thanks Frank.... 360 4x4 with a 4spd... shortbed, be a cool truck if it would stay running....