Car Symptom Question on Mitzi

I have a 1991 Mizibitzi Mirage that runs fine for about 1/2 hour then starts to sputter as though it does not get gas or gas line plugged up. When it starts to sputter I really have to give it the gas to keep it going. I have changed gas lines, fuel filter, spark plugs and distributor cap. This is still a problem at all speeds, and in all weather situations. What could be the cause? Failing fuel pump? failing oxygen sensor? Help.