car idle drops and car dies out when at stop.

What is your question?

when im driving the car its good but when i stop at a traffic light, the idle rpm's will drop to 500 then shut off. but if i hold the brake and put a little pressure on the gas pedal the car stays on.and when i got it home i let it idle for a little with it still in drive and it was fine, but the rpm gauge was bouncing just a little, like it wanted to die. I did alternator check(good) load test on battery(good), and the spark plugs look good also. so what could it be?