It is not your hardware.
I am kinda shocked with the memory you have that this is happening to you.
I was suffering from this for a bit. But lately the Fenris errors are all but gone.
If I can make a suggestion, look at your nVidia control panel setting make sure things are set the way you remember them.
The last 2 nVidia patches, 1 removed the global setting and made them default, and the 2nd overwrote all your program specific settings and set them to use Global value.
You can try rolling back the nVidia driver as LordTovek suggested to see if it works, but that would not explain why it is not working on your 6800Xt. Also I am on the latest nVidia Drivers.
The other small things I did, is I changed my page file managing back to letting windows manage it. I had it set to a specific size for another game, but it is not needed anymore.
I also changed the shader cache size in my nVidia Control panel to 10GB, but I set it to default inbetween. I don’t know that the size matters, you can set it to deafult or 4GB or whatever works for you. But I think changing it clears the cache and refills it. So it may seem slower at first (if you even notice it) but once filled it should be back to whatever performance you had previously.
I also turned off all RT, resolution scaling, I lowered my fog and shadows down one step from the max.
None of these will fix the issue, I believe the last Blizzard patch (or 2 ) made things a lot better, for me anyway. But most of these will lower the amount of memory the game needs…but again with the memory you have I am not sure how this is impacting you.
And I have been watching the performance tab on my task manager to see how that looks currently as I play.
One other thing I would ask is are your GPU’s factory overclocked? Or did you overclock them?