Custom Resolution Utility (CRU)

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You don't need to add it as a standard resolution because it already exists as a detailed resolution. The problem sounds like a 165 MHz pixel clock limit, which shouldn't happen with HDMI 1.4. Try importing the hdmi.dat file. If that doesn't help, the only workaround I can think of is to change the detailed resolution to 50 Hz.
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ToastyX, unfortunately, as I imported the hdmi.dat fite and restarted the graphics card, the monitor showed a message that no signal was beeing sent. Then I deleted the extension block, changed the detailed resolution to 50Hz and restarted the graphics card again, but I couldn`t change the screen resolution to 2560x1080.
Anyway, thank you for your time and for trying to help.