Need Help. Movie rendering (megui)

Hello guys

I just started testing some movie quality stuff and now i have a huge problem.
when i try to render the file in megui the quali becomes crap.
the uncompressed file i rendered with vegas looks quite nice.

I tried to use several rendering profiles (also some used by other moviemakers) but the quali does not improve.

I thought it might be the x264 codec but its the newest verision (r628).

Any ideas what may cause that problem?
maybe some drivers or other progamms?

Thanks