560ti SLI issues

Hi, I was hoping to find some help with a persistent problem I am having with my two 560ti cards I have running in SLI.

First, occasionally on reboot, the drivers will be completely gone, the cards are not recognized, and have code 43 errors in the device manager, at which point I have reinstall all of the nvidia drivers.
After I've fixed that, I will be unable to activate SLI. In the Nvidia control panel, it recognizes both cards, but will not let me enable SLI.

There are a couple threads I have found which address what seems to be the same issue, and it seems it was some Realtek LAN drivers that were the issue, and the fix was simply to roll back these drivers-
http://www.tomshardware.com/forum/276750-33-problem-win...
http://www.evga.com/forums/tm.aspx?&m=324313&mpage=1

However both of these threads are from a couple years ago, and I am not sure how to reproduce their steps. I built my rig at the beginning of 2011, and the problem seems to have been caused by an update that was pushed out a couple years ago. My Realtek LAN driver does not have a "rollback" option in the device manager, and I can't seem to find an associated download on the net. I would think that if this were a two year old issue, the drivers that my system shipped with would have fixed this issue but perhaps not.

Both cards and SLI had been working fine for the last year but I have had this issue on reboot three separate times in the last month, and SLI fixed itself mysteriously after the first time I had this issue, but hasn't worked since the second time i had to reinstall nvidia drivers on startup. I've checked all the hardware connections, and swapped the cards, ran one at a time and haven't found any smoking gun....

This is my first real PC after being a life-long mac user so driver issues are foreign territory for me and any help would be greatly appreciated. If more info about my system would help, just ask and I'll provide.