1) flash the BIOS again , & set to defaults
2) try a new driver set ( I didn't see you say you did this yet )
3) always possible 1 card is failing yet you didn't test it long enough for the issue to show up.? ( try running 1 card for 1 week, then the other )
1) Done it. Went and did it again to make sure by downloading the latest BIOS and burning it to a new CD all over again. Nothing changed.
2) I have tried different drivers, both beta and releases, same issue.
3) I thought about what you wrote here and ended up putting the card that I usually had in the second PCIex slot over in my missus` PC. It has been running there for the second day now without any problems. Her card, also a 480, is now in my PC with exactly the same issue.
You may also want to change PCIe slots as well, or just disconect and reseat, The board assigns resources, and IRQ's and a conflict can cause issues, I just recently was having issues with my 580's in SLI, my mouse was droping out at ramdom, and i was getting display driver fails, removing and reinstalling resolved it, but I had to do it three times, the second time I had some kind of conflict with my raid0 drives, and windows could not start. thgird time I have not noticed any issues yet, some IRQ's and DMA's do not like to share resources.
I have tries swapping around on the PCIex slots. Also disabled the HD Audio devices of both gfx cards.
I am not sure how to change around on the IRQs with Windows 7 and the EVGA x58 SLI. Any best practice there? In Windows the IRQ change is ghosted when I enter a resource in device manager.
With all this I am still leaning towards my initial gut feeling, that the problem is the EVGA X58 SLI mother board. I have also experienced ( just once during the weeks of swapping around on things) that there is a "no signal" from the graphics card connected to the monitor too.
But what seem to be the most consistent behaviour is that the second card disappear after a warm boot. So basically unless the system does a proper POST, the card won't come "online".
<message edited by tyfusius on Saturday, August 04, 2012 10:13 AM>