Do you have a electric fuel pump or are you still running the stock mechanical?
Either way I would say that it sound like a bad pump.
Some info about your truck would really help.
i replaced the mechanical pump. still doing the same thing. needle on gauge was flickering 0 to 60. engine will runs good but don't push it to long once the light goes on. back off throttle light goes out. starts up good. don't know what to try next. thanks