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Second monitor video keeps cutting out

I have an Imac Retina (5K 27"  late 2015) with an AMD Radeon R9 M395 2048MB  graphics card; running OS X El Capitan, version 10.11.6; and the second monitor video keeps cutting out. I had a Samsung 27" monitor on it since buying the computer, and this past month the video was going on and off. Thought it was the monitor going bad and replaced it with a Samsung 32" which worked fine the first 2 days. Third day the  picture comes on then goes out, comes back at varying intervals. Reducing the resolution keeps it on the longest but not completely. There are two thunderbolt ports on the computer and I have used both with the same results. Using tb to hdmi adapters and tb to vga adapters yields the same failing result. Using different monitors for the second monitor also gives failed results. I am wondering if the video card is going bad. Internet searches for this problem   revealed some saying they have more than one video card in their machines. Is that true? How many video cards does mine have and is it likely that the card is going bad or is there another issue? Any help or suggestions would be greatly appreciated.

最後更新：2017-10-21 22:56:45

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