285.03 opengl and multi monitor issues

[weltall]

I've an NVIDIA 8600MGT 512MB and I just got back home with my laptop after a while I was away and using a single monitor configuration only.
Now I've two monitors: the laptop one 1280x800 and an external hdmi 1080p monitor. While I'm using only 2d all goes well, but if I use firefox with hardware acceleration (opengl) or 3d games (even java based like spiral knights) I get random blinks on both screen - as they go black for an instant and then go back to normality - (seems to happen mostly when the game is loading object or textures or firefox is loading a page).
To note I use twinview to handle this configuration and as soon as I disable the external monitor all goes back to normality. likewise as soon as I enable it the issue reappears.

Did anyone else notice this issues? I've seen someone referenced a similar issue with vdpau but I'm not entirely sure it's related (they don't talk of a multiple monitor configuration)