Created attachment 292899 [details] emerge --info After upgrading to: 285.05.09-r1 my dual screen started to flicker randomly but for a long amount of time which makes it unusable. I tried the unstable version too: 290.06 but no better. Going back to the latest stable one: 275.09.07 Fixed the thing. I dunno where to start for helping you out but you can always ask some information (more than the `emerge --info`).
Looks familiar.
Created attachment 296381 [details] output of the nvidia bug report tool I tried again with the latest (and stable #394399) nVidia drivers 290.10 using xorg-server-1.11.3, xorg-drivers-1.11. The problem is still there.
Any ideas Jer? Can you retest with the latest X and latest nvidia-drivers?
Best bet since this is happening in the closed portion of the driver is to run nvidia-bug-report.sh and e-mail the output along with a description of the problem to linux-bugs@nvidia.com