Starting problems

I have a 1997 Ford expedition with 125,00 miles on the clock that runs very well. A problem has occurred recently where when I start the car it turns over perfectly and when I release the key and remove my foot from the gas it just dies instantly. I can keep the engine running by revving hard but then it dies This problem does not appear to be weather related (Los Angeles) and has occurred only in the afternoon and never in the mornings. I find if I leave the car ten minutes or so it will start. It has recently been serviced and the mechanic could not find the problem. Of course this problem only occurs intermittently, perhaps once or twice a week and it is very frustrating. Please help