Same thing happens to me with an 85 1.2 on a 5D and on a 1DsM2. Precise focusing wide open on the 5D and so so on the other.
Sergio - There's two different issues here. One is the lens' ability to focus accurately and the other is how well the lens performs optically on a particular body. In your case it sounds like a simple focus issue. If you have the option to drop both the lens and the body at a Canon repair center they can calibrate the body to the lens and you'd be good to go. On a 1DsM2 I'd recommend selecting focus points manually (set the camera to use only the center and the outer 9) and then when shooting wide open on a 1.2 lens only using the "+"-type focus points. Assuming your subject matter permits this, things like portraits, etc.
With the 85/1.2 I found that the 1Ds and the 1DsM2 bodies I had all needed a tune-up by Canon to be 100% accurate with the lens wide open. On a Mark III body you can do this tune up yourself via the microadjustment settings in the camera menus and running some tests.