Asus GTX 750 Ti Causing Problems

[Obama]

I recently went out and bought a GTX 750 Ti along with a Lepa 600 Watt PSU, ever since I installed them, my computer has had gray screens after I turn on my computer but I can see the mouse and move it around so I know it isn't the PSU, I always have to restart the computer, I've tried waiting five minutes but it never goes away. Also, when I play GTA IV, every so often it will go to a black screen for about 10-15 seconds while still playing sound, and when I play GTA V, After playing for about 10-20 minutes it will suddenly go to a gray screen and an error will pop up saying there was an error and that I needed to restart the game. None of these problems were present before I installed the graphics card and I cannot return it because I've already sent in the rebate.