2002 Mazda Protege ES starting problem

My 2002 Mazda Protege ES occasionally turns over without starting regardless of climate conditions. After waiting about 15-20 minutes
it starts. This happens randomly. It would be fine for 1-2 weeks and then it happens again. My mechanic is stumped. He's cleaned the throttle body and replace the air intake hose. It runs better but I still have the same problem. He doesn't want to keep replacing parts without knowing the exact cause. Any help is appreciated. James
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