What's wrong with my car?

I have a 2003 Honda Accord LX ...just had the starter replaced and battery is bout 7-8 mos old...when the car is cold it starts up fine ...no problem...after i have driven it(like just going to the grocey store bout 3 miles) and the the engine gets warm.. the car will not start..it won't even turn over...but yet everything else functions in the car(lights, pwr door locks, radio, etc) ... i have taken it to the mechanic several times ...he has checked it and told me there is nothing wrong with the car??? He said he drove it around and parked it and shut it off and it starts right up again...but he only drives it around the block...But obviously there is something wrong??? Some one told me it might be a sensor or ignition cylinder??? Please any Help is appreciated!!