2000 Toyota Camry Stalling and Engine Light

My 2000 Camry's Check Engine Light recently came on and I took it in to my mechanic, but he could not find anything wrong with the car after connecting to the diagnostic equipment. I drove the car away from the mechanic and the light stayed off for about an hour than reappeared. Also, when I slowed to a stop at a light my engine dies. It started right back up and I continued driving without a problem until I slowed again. The next day, the car started without a problem and the check engine light was on again. I drove the car on the highway for about 30 minutes and when I slowed at the bottom of the exit ramp, it dies again. I started it up and it drove without a problem until I slowed again. I am taking it back to the mechanic, but wondered what you thought might be the problem.