How do you know if you need a new fuel pump or filter?

I drive a 03 Mercury Sable LS Premium Duratec 3.0L with about 105,000Miles on it. The car has been bucking pretty bad about a week ago but a quick check of the plugs revealed large gaps. So we bent them back to proper measurement for the car and problem seemed to be fixed for couple of days. Now the problem came back and it was acting the same for a little when earlier today there was a loud humming noise coming from somewhere in the car. I figured it was coming from the back of the car, but you know how difficult it is to diagnose sound when your driving. This was when I was climbing a hill but when I went down the other side of the hill the humming stopped. Then, every now and then, when I come to a stop the CD disk changer stops, interior and exterior lights go really dim with the music kicking back in but at about half the volume it was at. I have no idea whats going on with this car. Could this at all relate to the fuel pump or fuel filter?? Thanks for any help. -Andrew