I was trying to upgrade my son's GTX-750ti with my old EVGA GTX-680 SuperClocked card, but it made the system so unbootable that I couldn't even open the BIOS CMOS settings utility.  The system would randomly hang at the BIOS splash screen, list of attached drives, and randomly reboot - without ever trying to boot the OS (Win10-64).  Eventually I just had to give up, and go back to the original GTX-750ti video card - which got it working instantly, but I have no idea why...
 
The system is built on a Gigabyte AMD A3+ motherboard (GA-78LNT-S3 at it's latest 2014 BIOS update) running a 3.8GHZ FX-4300 4-core processor, with 8GB DDR3 memory.  Yeah, I know it's a moderately old motherboard, with only 1 16xPCIe slot - but I couldn't think of any reason it shouldn't be able to use the GTX-680 successfully...  The latest and greatest GeForce drivers were already on, but that's probably not relevant since it never got close to actually attempting to boot the OS.  We tried powering the system off and restarting the boot attempt multiple times, and could see that all the expected HDD's and SATA devices were recognized by the device listing that would sometimes appear before the system totally hung.  Even from the CMOS splash screen, none of the key sequences were recognized or acted upon to enter the CMOS set up, or other BIOS utilities.
 
I was trying to remember if my EVGA GTX-680 needed a BIOS extension or anything to work, and if I had to do anything weird to the BIOS to install that - but could not find any reference to anything other than the latest GeForce drivers on the EVGA product support page...
 
So does anyone have any idea why this GTX-680 fails to work with the Gigabyte mother board I was trying to use it in?  I really wasn't expecting any problem when I put it in, but was surprised how it made the system totally unbootable.
 
Thanks for any thoughts, explanations, or suggestions anyone may have!

Whoops, yeah I misread what my son wrote down when we were checking the BIOS level - you listed it correctly as the GA-78LMT-S2.  Last BIOS update ("F6") was in January, 2014.

Thanks for your quick responses and interest!
 
Yeah the GTX-680 was working o.k. when I took it out of my big 2013 vintage i7-3930k system last fall, after a liquid cooling failure (that leaked all over the CPU socket due to a cheap OEM pump) forced me to totally rebuild that system (into a slightly better i7-5820k with an 8GB MSI RX-480).  I'd never overclocked the 680 - never needed to...  I know the GTX-680 was working fine even after the original liquid cooling failure (since I got the damaged system to boot once and had it up for 6 days after that - before even rebooting became hopelessly unreliable), until I pulled it when i rebuilt the old system.  It has been sitting on the shelf for almost a year.
 
The research I did for getting my son's "new" FX-4300 gaming computer to work better indicated that the 2GB GTX-680 should run rings around the 2GB GTX-750ti he had (that we'd moved over from his old system) - so thus the interest in upgrading his system to it...  We put in a new Thermaltake 500w power supply today as part of that process, to replace the unimpressive and suspect "Venus?" power supply provided by the OEM, which had tons of negative comments on Amazon and for using the GTX-680 didn't even have the extra power plugs for providing extra voltage to big video cards. 
 
I figured that the new Gigabyte system was sort of seeing the 680 card o.k., since we'd see the BIOS splash screen and occasional listing of the attached SATA devices appear through its initial VGA output.  Was baffled that I couldn't even: (a) get consistent results when I hardware reset the system - other than it would always hang on one of two initialization screens, (b) couldn't select or enter any of the CMOS support utilities - even when I had the splash screen you'd normally access them through up, and (c) couldn't get the hung system to restart again without a hard power off/on.

680 Likes Legacy GPU Bios over UEFI GPU Bios Setting.