My copy of XIII works with the nVidia driver 66.83 but not with the latest v70, can anyone help me on why that is?

There is a patch available for the game (LINK), have you installed it already ?

Yeah. It seems like only the ver.66 driver works.

I'm playing it with the 71.89 myself so I'm not sure the nVidia drivers are the problem. I do get weird slowdowns as if there was some "bullet-time" activated, it doesn't seem like it's the VGA card lagging.

What seems to be the problem when you launch the game ?

The start up screen with the Ubisoft logo freezes up and sometimes doesn't even show up, i.e. it's just a black screen I see, maybe it depends on what Geforce card you have as well, cos ver.70 is really aimed at Geforce 6 cards, so it does nothing to improve FX series or 1, 2, 3 and 4