I have been trying to get two Palit GTX 275 cards working with SLI connection on an ASUS P7P55D Pro motherboard, and they seem to work fine as long as I don't reboot my computer: if I have enabled the SLI via NVidia Control Panel, I get a bluescreen during the reboot and my Windows 7 won't start; and otherwise, if I haven't enabled the SLI, Windows will start but the SLI enabling option in the NVidia Control Panel is gone and the only way to get the SLI configuration working again is to uninstall and then reinstall the graphics card drivers.
Is this a common problem, or is there just something wrong with my hardware? Any ideas how to tackle with the problem?
I am not sure if you will be checking this post as it's a little outdated. But I had the same problem as you did. What fixed it for me was rolling back my Realtek LAN drivers (as odd as that is) and installing the original one's from your mobo's driver page.
Apparently some computers are having issues with windows realtek driver update.