Jagged edges

I am getting really frustrated by illustrator, as it keeps showing jagged edges. I checked a lot of posts about this and I checked most of what i could find, it's definitely not the anti-aliasing thing.

Also, this one in the same file, does not have that problem whatsoever:

http://i133.photobucket.com/albums/q75/Scarphion/test2-02_zps82db1f9c. png

It is also visible if i do not put the dark bg behind it, it's just so its clear. The lower part of the tree is normal, but then it gets jagged and ugly, just like the grass beneath it.

Any ideas how to get rid of this? It really bothers me when working.

I work in CS6 btw.