I own that same card, the Sapphire HD5870 Vapor-X. Based on my own experience and another forum post that I read somewhere, the card is causing the problem. The only solution that ever worked for me out of a huge number tried was to get a second video card. I went with an inexpensive Asus HD5850 TOP, which works in Crossfire mode with the HD5870. If I put the Vapor-X in the primary PCI-Express slot, the monitor connected to the displayport adapter loses its link. If I instead put the HD5850 in the primary slot, there are zero issues. You need to either return the card if possible or get a second one. In my case, everything is great now because games run more smoothly in Eyefinity, and my fourth monitor is doing more than collecting dust.
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