SLI only sometimes works help!

[xninelives]

Ok so I've gotten SLI to work. Have two GTX 470 SLI on a P9X79 WS board by ASUS (Suppports Quad-SLI). I've downloaded the newest drivers. Also downloaded all drivers for all the other hardware in my system. I've tested that the second card works by inserting it in the first PCI-E slot. And yes I installed the SLI bridge that came with my motherboard. Maybe the SLI bridge is faulty? I got a Corsair AX850 watt so I know I have enough power. As I've said I've ran SLI before it, and it works fine. Its just that sometimes when I start up the computer it doesn't have it in Device Manager/Nvidia control panel. Thanks to all that help!

 

[xninelives]

Ok I've noticed that whenever I install a new OS it seems to work. I only install the minimum drivers. Chipset, and GPU (Nvidia of course), I have a wireless card so no LAN drivers are required. No other software has been installed. I've done this on the past 3 installs. So as I've said it works for the same boot or boot that is done right after I install NVidia drivers. Then later I turn my computer back on with NOTHING changed and it has no option for SLI. Also is not in device manager. Please can I get some help, suggestions even. I've reinstalled my OS 5 times in the past 24 hours.