Sigma 30mm 1.4 hit or miss - user error or lens ?

Sometimes, when I shoot wide open at f1.4 with or without flash using AV priority, 40% of the time I get a soft focus right smack dead center where I pointed using manual centered focus ring... I thought, most of the time this shot is a piece of cake until I look at the images on the PC and somewhat soft.

I decided to use this chart maybe not accurate test of AF issue but what do you guys think?

Is it me or is it the XSi body AF issue? I did set the camera to 0 sharpness...

Test shot at 30mm f1.4

HTTP response: NOT FOUND | MIME changed to 'image/png'

HTTP response: NOT FOUND | MIME changed to 'image/png'

Maybe the Av is choosing too slow of a shutter speed? What speeds are your soft images shot at?

Also, Maybe provide some samples with exif?