Does the PC function correctly with the graphics card removed and all other original cards fitted? If so five possibilities come to mind:
Is the card seated properly?
Is the card faulty/overheating? Check the fan is running.
Have you got corrupt drivers? Uninstall/reinstall to see if it corrects the problem.
Is there a software conflict? Old drivers?
Is there a memory problem with the PC attempting to access card RAM or VV?