The most common cause for this symptom is lack of fuel pressure. The first thing to do is measure the fuel pressure. It should be 55-60 psi engine running and 60-66 psi engine off key on. You can sometimes gain 3-6 psi just by replacing a clogged fuel filter. Above that you will need to replace the fuel pump itself which is located in the fuel tank. Remember to releave the fuel pressure before disconnecting the fuel lines.