1990 Geo Prism engine cranks but wont start

1990 Geo Prism mileage: 75,000. My car will not always want to turn on, when I turn the ignition switch. It wants to turn over, but it will not. It generally takes about ten to fifteen minutes of me trying to turn it on, before it finally does. Some days, it turns on the first attempt. I've taken it in to an auto mechanic, but they cannot get it to stall. I think it could be the fuel pump, because here lately it has been using a lot of gas. What do you think? If it is the fuel pump, are all fuel pumps about the same in quality? Thanks.