My monitor keeps shutting off and won't recognize my video card

I used to be running a GTX 275 but then the monitor started turning off at random times. After that happened a few times the graphics would get really messed up when I restarted my computer. There were a bunch of vertical line going through the screen. My computer also wouldn't recognize the video card. I am using evga precision and it was NOT because of overheating. Other people have had the same problem and they all fixed it by getting a new video card. So i borrowed a 9800 GT from a friend and and downloaded the drivers and everything. It was working well for awhile and then my monitor shut off randomly again. It's not happening as often as it did with my 275 but it still happened and I've never had it happen before. Please help!