Waiting at a red light the car sort of jumps?

I have a 1999 Toyota Camry with 116000 miles. Sometimes when I come to a full stop, or i'm waiting at a red light the car sort of jumps, and then it turns off. Sometimes I feel when its about to turn off, so I put it in park and give it some fuel and it doesn't turn off, but I have to be fast enough. I've changed the cables, spark plugs, and air filter. I change the oil every 3000 miles with full synthetic. I changed the timing belt at 105000 b/c a mechanic told me that this would take care of the problem, but so far no luck. It cannot be diagnosed b/c it does have a light engine on.