Alastairhaig, Is unit 4 years old? Could be problem with units switch-mode-power-supply if same thing occurs when connected to fully functioning different pc that has all screen saver and power save features disabled. After screen goes "black" do you still see the WIN-desktop screen and icon's if you hold a bright flashlight at different angles about 1 inch from the screen? SEE = power supply in "partial shutdown" mode. NO SEE = smps in "full shutdown" mode. Do some reading in my fixya profile about "black screen of death" repairs. also read FIXYA profiles of "budmrtn" "buddymc" and other fine profiles of technicians here on fixya. You will truly be amazed when you visit YOUTUBE for video clips of owners/technicians who save their cold cathode fluorescent lamp, back-lit lcd monitors from the "trash-heap"!!! After you do your research please return here and rate this response and possibly do additional comment. 12fixlouie