Sounds Like a driver problem I have dual gt9800s and the drver for them as well as tour 9600 gt. Vers 8.15.11,8627 which can be had at the nvidia website.
Long story to start with.
Hey all, over the past week ive been having major problems with my 9600gt card. It all started when i have to pack up my pc to move house. When i reconnected it all back up i had a black screen. So i took off my vga to DCI adapter and plugged the monitor's vga cable into the normal vga socket. When i managed to get the thing on vista couldnt detect my nvidia graphics card, and instead was using the "Standard VGA Graphics Adapter". I couldn't fix it for a few hours so i just reinstalled windows to save myself more hours of pointless hassle. When windows installed it could detect my nvidia graphics card again. But i had fewer resolutions available including the standard 1680x1050 resolution i have always used, though i did manage to find this in the nvidia control panel but 1680x1050 wasnt available as an opion in any games i tried including world of warcraft call of duty etc, meaning everytime i tried to play something my res would reset to whatever the game was using. I also forgot to mention after reinstalling windows i switched back to my vga>dci adapter. I have also had some freezing problems. Now and again my pc will lock up and need to be restarted and its making my pc pretty much un-useable. My drivers are all up to date and ive never had any problems like this previously and ive completely run out of ideas on what to do.
Apologies for the wall of text but didnt want to miss any info
Now for the short version and some things ive noticed that may help diagnose the problem.
When the nvidia card is working, it will sometimes make the pc freeze and need to be restarted. This is completely random and not when im doing something that could be even the slightest strain on the pc, even sometimes during startup.
Uninstalling the on-chip from the device manager and restarting with the vga in the DCI slot sometimes fixes the problem.
Sometimes (usually after being off while i sleep) it will fix itself and will work when i turn the pc on.
So far though it has repeatedly froze my pc or randomly stopped working and it's really stressing me out.
Thanks in advance for the help and let me know if you need anymore info. (though keep in mind im not the most knowledgeable when it comes to technical computer stuff ^^" )
Again apologies for the wall of text