It can only be a few things. If you have to guess at all, start with the fuel filter first if it has been a while ago that it has been changed. If it is full of dirt, pull the pump and replace the sock next. The next item to be the culprit is the pressure regulator. Fuel pumps either work or don't most of the time. It should only be replaced if everything else checks out. Best bet is to obtain a pressure gauge and verify you have or don't have the right pressure.