Been running windows 8.1 and an Asus GT520 graphics card for some time with no problems. After removing and replacing the card I have a problem however - PC boots as normal, with the blue Windows logo appearing - then suddenly the screen goes blank and monitor goes into sleep mode, and stays like that until I do a hard reboot. Can log in via safe mode; device manager shows GT520 is working and drivers are up to date. If I remove GT520 and it's drivers in device manager and reboot I can log in using chunky default graphics, but once Windows identifies the card the same problem reoccurs on reboot. Any thoughts?