Thanks.
Some time ago I've read that there was a problem with the 90mm Sonnar but it was usually solved by Contax. The camera and the lens were sent to Contax and it made calibration. At present it seems there is no solution.
I have some sharp or almost sharp photos made with the Sonnar - most of them made at f/4 and f/5.6. So, the lens is better than nothing but it isn't what I expected of it.
It's a blistering sharp lens! OK, not taken on a G film but evidence of rendition
Both shot on a Panasonic GF1 and Contax G 90mm wide open (manual focusing!)
http://farm3.static.flickr.com/2780/4449850047_de69826913_o.jpg
http://farm3.static.flickr.com/2753/4424360026_2e989ce165_o.jpg