Take the fan part off, and look on the front and back..you should see a sticker. Carefully take the sticker off, removing as little bit of stickyness as you can. Then you should see a little hole with a nylon washer inside. Squirt a small bit of bearing lubricant in there (WD-40 will work too), and then wipe the surface as clean as you can. Use a little bit of rubbing alcohol to clean it so the sticker will stick, but don't get any in the hole.
Replace sticker. If the sticker doesn't want to stick, then just wait a while for the surface to evaporate, then clean it again, and cut out a little piece of ducttape.
Should be good as new.
I had an old hard drive cooler, and it was so bad that you had to push hard with your finger to turn the blades. I did the above ^^ and it worked really good, moves a lot of air still.
EDIT: Actually, before you do that.. how many amps does your 12v rail have? When a fan starts, it takes something like double or triple the amps to give it the intial start up. Could be that your PSU doesn't provide the power to start it. I base this theory on the fact that I've never heard of that make of PSU (which doesn't mean it's junk, maybe I've just never heard of it) and the fact that your computer sometimes doesn't start, lol.
But try cleaning it and see what happens. Maybe because it's dirty, and it doesn't start sometimes, maybe it's robbing all the power and then your computer doesn't start? Who knows..try cleaning.