[ubuntu] Screen fading problem on laptop

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http://ubuntuforums.org – My apologies if this has been posted before, but I have searched and haven't found any obvious answers. I need obvious - I'm very new to Ubuntu and Linux in general, and have installed Ubuntu 9.04 on a Toshiba Portege laptop. For the last few days it's worked fine, and yesterday - without having installed anything or changed anything as far as I'm aware - it's suddenly started fading the screen in and out. I've read various posts online about dodgy power connections, etc, but this doesn't seem to correspond. The fade is fast, a second at most, and it goes right down to black (presumably not just a screen brightness issue), before fading back in again equally quickly; sometimes it stays black for a moment and then flicks straight back to 'on'. If I'm doing anything, it's stopped until the screen returns. If I'm dragging something at the time, I have to reselect it and drag again afterwards. It may be a hardware fault, but it doesn't seem to happen if I've got a menu open, so I suspect it's a setting somewhere. I have noticed the screensaver flash up a couple of times, so I've tried to disable the it using a couple of console commands I found online, but the fade problem still occurs. It happens on battery and on mains power. The fade occurs every couple of seconds, and it's very very annoying. Would anyone have any ideas as to what's suddenly gone wrong? Thanks for your time. (Hardware)