I am experience intermittent frame stuttering with my SLI GTX 580s on Windows 8. The frame rate is smooth 90% of the time but will randomly hang and stutter every few minutes. I am on the latest drivers and have D3 installed on an SSD with an i7-2700k and 16gb of RAM.
If I disable SLI, everything runs smoothly. I have tried with vsync on and off and have the same effect.