When I first start my car for the day, it cranks fine, revs rise... then drops very low and car shake, then rises a bit more then drops past normal idle again but not as bad, then rises again and then settles.

Same happens if im going from P into D.
So in other words, if the car is cold and its dropping down to idle revs it will drop below idle speed and nearly stall.
But this only happens when cold, once it warms up for a bit, its fine.

Im thinking something along the lines of the ISC?