1995 Chevrolet Blazer won't start

1995 Chevrolet blazer mileage: 44,000. Blazer won't start at times at other times it does fine. Mechanic looked at car twice but couldn't get a read on the problem because it started every time he tried when it was racked on the computer. The engine turns, I can hear the fuel pump turning on and off, everything else in car works fine but it simply won't turn over sometimes when you crank it leaving me stranded where ever I may be. If you leave it alone for a few hours after initial fail, it will fire later