pc freeze after pink crosses on screen

I have a custom built pc and every time I boot up my pc after a day or two, the screen turns to pink lines and then it freezes. However, after I restart the computer, the lines don't come back and the pc won't freeze. It only happens whenever I turn on the pc after a very long time. Here is an image to see what happens. The last thing I did was add a 4gb dual channel memory kit. Please write back with any suggestions, thanks.

 

First, try another monitor to be sure it isn't a monitor problem. If the same thing happens then it might be the video card. I'd mention video driver but since the problem only occurs after the computer is off for a while, it seems more hardware than software.