Yea shaun o said it perfectly. Now when it turns off, if you take a flash light to it, can you still see your desktop? If so then its just some bad capacitors that you can usually find in a kit that is sold on eBay or Amazon. Me because i fix stuff like this on a weekly basis i always have a vareity of caps laying around and I only use high end japanesse caps.
http://www.badcaps.net/ can tell you and show you a lot and for the most part stick with Nichicon Nippon Panasonic Rubycon Sanyo brand Caps. They are the good brands.