idle irregularities

[SinceByMan]

I searched and could only find things that were similar but not the same so here goes.

In park, my car idles at 900 but in drive at 700-750, sometimes when Im at a stoplight, it drops to 500 and when I leave the light, it hesitates

If I turn the A/C on, the idle should increase shouldnt it? When the car is running at 750 rpms, and I turn on the A/C, the idle jumps up a bit, but when its at 500, the A/C works, but idle stays low, car shakes a bit.

I just want to know, why my car goes to 500 and is it hurting the engine...i.e. not enough oil pressure or something?

[Queamore]

Sounds like a vacum leak to me. Listen closley to the engine see if yo can hear something sucking in air. or spray some wd40 Around the engine like let it mist up and the engine should bog a little bit or you'll see were the mist is being sucked into. If it's not that the check the computer.