Hmm... I would uninstall your video drivers completely. After you do it, restart your comp and THEN install the drivers again. If it still randomly restarts, check to your gpu temperature with the riva tuner, might be overheating for some reason. If that doesn't work, test your graphics card in another computer and see if it runs stable. If it does, then you have another problem on your computer my friend (could be your psu, mobo, might have to reseat your cpu, make sure to run a memtest as well...). Thats all I can think of right now.
1. Uninstall drivers, restart, resinstall drivers
2. Check your GPU temperature using nVIDIA Riva tuner
3. Try the card in another computer, if it works, step 4!
4. Assume there's something else wrong with your computer
5. Make sure you took off any plastic covering or anything that was on the GPU to protect it.