For whatever reason my computer crashes when I have three cards running at once. With three the computer will cut off automatically after about 10-15 minutes, and with two cards running, and one sitting idle, the computer will completely freeze up in about an hour. Is there anything I can do to fix this? The only fix I've found is to unplug one of the three cards, then two can run just fine.

This is the power supply I'm using. It should be able to handle the job since, with three cards running, only 950w is being used up, which leaves over 200w of overhead. Also, as for the RAM, I ran a memory test on it at advanced settings...nothing came up, so the RAM isn't the issue.