Start the PC in safe mode, uninstall all GPU drivers , meaning, programs and drivers from ATI and Nvidia. Use Driver-sweeper for remnants of any of them.
Then reboot the system, but before you do that check if the new card is properly seated in the PCIe slot. Make sure all Power Connectors are properly on.
Then install the latest drivers for the 520 from the Nvidia Site, make sure it's not the Beta Driver.
Check to see if problem still occurs. Even if it doesn't download a Temp Utility to make sure the GPU is running at normal temps and not way above the usual at idle and load.