2002 Mazda Protege ES starting problem

Hi,

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