Black screen on inactive clients

I recently built a 980x Extreme gaming rig that blows my socks off. Now I am a little disappointed as I am having graphics issues.

I have 2 x 480GTX Gigabyte Video Cards. They are not SLI because I was told if I was going to multibox MMORPG's to set each up with it's own display. So each card is feeding a LED monitor.

I ended up getting a 2nd card because I was running multiple copies of an MMORPG and was getting some graphical glitches when I switched between the characters. After the 2nd card was installed these went away and all was fine for about 36 hours.

The problem I'm having right now is that when I game, the clients over on the 2nd display end up black screening between 2 and 5 minutes into playing, they still respond as I can issue keyboard commands and see the actions performed on the character on the 1st display. So I'm really stumped as to what is causing this.

Are you running windowed mode?

Yes all in windowed mode.

Power supply big enough? Or the cards are running hot.

I have a 1200w power supply so plenty there. Also turned my GPU fans to 100% and the Temp on neither card rises above 40c. So can't be heat either.

Faulty hardware? Could try swapping the cards around or running with a single card to help diagnose the issue

How can it be the cards?

If I run a single card it does the same thing. The "inactive" windows will eventually go black.