I can suggest the two possible solution for you, which are following:-
- Go to manufacturer's site and download latest drivers for your video card
- Get to safe mode by hitting f8 on boot up, uninstall all video card drivers
- Reboot the computer into safe mode - Install fresh video drivers
- Restart system and let it start Windows normally
Check your BIOS settings and see if you can lock the PCI bus frequency and set PCIe frequency to "auto".
It sounds like either your video drivers are corrupted or your overclock is over stressing components on the PCI/e bus. Sometimes an overclock can bring problems even days or weeks after having been without problems.