Posterization problem with 550D

hello everybody !

Since I have my 550D, I've noticed some posterizaton, sometimes very visible, mainly in deep blue skies. I had never noticed such important posterization with my 350D, so I was wondering wheter it could be a problem from de sensor ?

I mainly shoot in JPEG fine, so I've tried to shoot RAW but it doesn't fix totally the problem. I use a F2380M screen (non calibrated) but tried 3 others monitors with the result.
I've already updated the firmware. Have you experienced the same problem ?

Here are some examples (original JPG and raw):
http://vuachet.delphine.perso.neuf.fr/bd/IMG_1724.JPG
http://vuachet.delphine.perso.neuf.fr/bd/IMG_4733.JPG
http://vuachet.delphine.perso.neuf.fr/bd/IMG_6957.JPG
http://vuachet.delphine.perso.neuf.fr/bd/cpex04.jpg
http://vuachet.delphine.perso.neuf.fr/bd/cpex04.cr2

--> bad sensor ?
--> bad screen calibration ?
--> JPEG artifacts (but why are the shots from de 350 correct ?)

Thanks a lot for your advices