Powerbook Screen Goes Black

I've got an older 15" powerbook running Mac OS X 10.3.9 (i think). Lately my computer screen randomly turns itself off, the whole think going black, like it was asleep or off, but the computer keeps running just fine. Like i might have itunes playing or be burning a disc and that goes on just fine. The computer itself stays running. What I've been doing is putting the computer to sleep by closing the notebook, letting it rest for a few seconds and turning it back on by opening it back out. This fixes the problem for anywhere from 5 seconds to indefinitely, until it happens again. Lately its been happening more. The computer is 4 years old, but the model itself, probably 5. Any ideas?

Thanks

Black screen details

Thanks for the replies. The computer doesn't actually put the screen to sleep, in the sense that I can't un-black the screen without closing the machine, which seems to trick it into turnign the screen back on when I reopen it. I tend to have a video ipod plugged in.