power loss need help
 

ok so I just picked up a 95 z28 auto. it starts up fine. it idles fine and for the most part it drives fine. however from time to time and at random it I lose power and get a kind of knocking sound but will go away if I power through it. I cant tell if the knocking sound is coming from under the car of from the motor I know that the car does have a minor exhaust leak by the header any help would be appreciated. I also know the car was recently ran completely out of gas replaced the fuel filter. and still having problem

test fuel pressure. sometimes the last straw on a fuel pump is running it out of gas. pump runs all the time and when tank is low the pump is not covered with gas. the gas acts as a coolant to the pump and without it covering it overheats. also may want to look at the catalytic converter to see if its plugged.