I also have this problem on occasion. Except with my car the idle is a bit higher than normal and rougher and occurs both at the stoplight or when shifting gears. A blip of the throttle usually works to calm it down at the stoplight.
So it sounds like I need to check the throttle cable slack, gas pedal pivot point, and clean or replace the idle air control valve.
Mine had similar symptoms - turned out to be a frayed throttle cable at the throttle body end causing the throttle to stick open slightly.
If it is this and it gets worse then eventually the throttle will stick open and once you switch the engine off and try to restart it the fuel pump will run, the engine will turn over but it won't fire up.
