Fran, have you considered that it might not be mistiness, but  light reflections? Nearly everything reflects to some degree and looking at your checkerboard, the lightness towards the back looks more like it's catching the light.

It might be an idea to post the actual Carrara file on this thread so some of the pro's can take a look at the scene settings. Carrara is a very good renderer - almost photographic - so mistiness is much more likely to be from the way you have the general atmosphere and lighting set up than a problem with Carrara. And as for C5pro, yes, give it a try and then go up to C6 pro :)