rpm drop

i was sitting at a light on my way home from work and my rpms were consitantly dropping from the idle 700 to about 200-250 and the car would shake really bad. it would only stay at 200 for a split second, and then jump back up to 700. it was doing this cycle about every 15-20 seconds. i took it out of first and put it in nuetral and it still continued. im sure somebody has discussed this before but i could not find it. any ideas on what might be causing this?

Egr.

i dont really remember, but wasnt that something that was fixed on the recall, or was it the intake maniflod screws or something like that? any thoughts on if it can be fixed and how?