problems with EOS5D + Pentax FA 645 35

[bernhardmarks]

hello
i work with a EOS5D + Pentax FA 645 35 lens (architecture). problem: i get some color-stripes (like rainbow) in the sky. see attachment.  if i procced from RAW to tiff it´s much better, but not perfect. i don`t have such problems when using my other lenses (tamron, canon...). what is the reason? what can i do? uv-filter?

regards,  bernhard