Have a dual Samsung Syncmaster flat screen monitor setup on a recently purchased 2008 3.1.(Same monitors worked fine with my old 2006 1.1) Everything boots fine but after 20 minutes the video goes black. I have to shut the machine down then boot again to get video back. Monitors stay powered on throughout. NOT a sleep issue I don't think as I'm actually working on something when it happens. I tested both monitors on their own and on other computers they work fine and don't lose video. Just got this computer I'm suspecting the Graphics card?
Specs of machine in signature
ATi Radeon HD2600 XT 256 Graphics
Apple Mac Pro 2x2.8 GHz. 3.1 Intel Xeon Quad-Core
20GB 667Mhz DDR2 FB DIMM RAM, 5 SATA HDs internal
ATi Radeon HD2600 XT256MB Graphics/2 Allegro USB PCIe cards
Mountain Lion/El Capitan
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