DX9 graphics glitch -- GTX460...ideas why?

My (admittedly aging) GTX460 is showing some odd artifacts with fire and smoke since MoP released. I haven't bothered with it but DX9 is plenty "shiny" for me were it not for this glitch and it is significantly better performance. Anyone have an idea what might be causing this or what option might rectify it? Doesn't happen in DX11 mode. I don't recall it happening in Cataclysm, either. NVidia driver is 344.11, though it's been doing it for quite some time (296.xxx builds). I think 270.xxx range of drivers didn't give me any issues, though I'm loathe to go back quite that far.

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That is certainly interesting and I can't say that I've experienced anything like that before. Does it happen when not using Inner Space? Or when playing solo?