Help please - Shadow issues with 350D

[AussieBloke]

I went to process some photos I took from a trip to Amsterdam recently, and when I look in the shadow areas on a lot of my pictures I see some dreadful artefacts - namely colour noise and posterization. I went back and looked at some other pitcures that I have taken recently, and they show similar issues. I think this may be an issue with the camera (maybe the image processor?), but what do you guys think? I've attached one example with a 100% crop, shot in RAW with EF-S 17-85mm IS lens, ISO 200, f6.3, 1/160. Shot in Adobe RGB, converted in Camera RAW. Then coversted to 8-bit and sRGB and saved to JPG. No other post-processing has been done. These artefacts also appear at the sensor's native ISO 100 sensitivity from what I have seen.

Any help is most appreciated.

Thanks,
Sean