i'm getting this weird interlacing/line problem..?

hi i keyed out some footage and precomped it, brought into a new comp with a
background "room" in it but i'm getting this weird problem with the
footage looking like garbage, i'm still working on the video but what can this
problem be? it almost looks like an interlacing issue or something, but it's
these lines across the whole footage. all layers are 3d, and i have an ambient
light layer in case that has anything to do with it. you can see it a lot in his
arm near the end.

Not really much to see in that YouTube video that could be interpreted as
encoding artifacts. Can you post a still frame? What you describe could possibly
be pixel aspect ratio issues which sometimes occur when mixing different ones of
them on 3D layers. You should also check your key. It's possible that your Alpha
is not 100% transparent and causes some unnecessary extra blending operations
that screw your stuff.