Idle problem I30

[pghobbsx]

When I first start the car in the morning the idle is fine. After some driving when I stop the car at a light the idle will jump up and down from 800 to 400 rpm. It does this every couple seconds while I am stopped at a light or a stop sign. It never dies, but the idle goes down so far the car starts to vibrate a little like it wants to die. Any idea's would be helpful......