Car wants to stall when I put it in gear

[ChunkFunky]

I know it's been asked a million times I'm sure but here goes again.

This is an AOD and it idles at about 1000 when in park, but then when I put it in gear the idle dips way down to like 500 and then the computer adjusts and gets to about 800 and idles fine there. I was under the impression that when you set the idle the computer will keep it at that idle at all times weather in gear or otherwise. When I PUT it in gear it starts to stumble and then it goes back to 800 and runs fine there... though I find if I put it in reverse it will stumbler harder, and infact will stall from time to time. It's not a big deal, though it was never like this before I did the heads and intake and all .
-as