My UZ2715H (just bought btw) kept dimming every few seconds, on multiple PC's too. Screen saver off, high performance power option, sleep mode off, used HDMI, DVI same story. Read the Microsoft answers to run SFC (their standard response, everything is corrupt files, even on fresh installs). While pulling my hair out tried one thing and it worked.
Dell's Display Monitor software: turn OFF manual mode, turn ON automatic. Choose any display temperatures you want. Fixed the problem on every machine. Hope it does for anyone else out there.