PowerBook goes to sleep unexpectedly

I have a 17" PowerBook G4 I got about 8 months ago. It's been absolutely wonderful, until today.

I was just doing normal things on it, and it suddenly went to sleep while I was working on it! So I pressed a key to wake it back up, and a few minues later it went back to sleep again. Now it's been doing this all night.

I thought maybe it had overheated (even though it wasn't particularily hot) so I shut it down and let it cool completely, but when I turned it on, after about a half an hour it began going to sleep again. While I'm even doing things on it.

I have it plugged in to AC as always, but my battery is fully-charged anyway. It's set to never go to sleep no matter how long the inactivity has been. I haven't installed new software in at least a week. I haven't done anything out of the ordinary in a long, long time. I always install Software Updates when they become available.

I've created a new user account (I'm on it right now) and it hasn't gone to sleep yet, but it hasn't yet been like an half an hour, which it seems to take to start acting up. I'm HOPING it won't go to sleep on this account, so I can at least know it's a software problem and not a hardware problem. I NEED this computer over the next few weeks as I have some VERY IMPORTANT video production projects due.

Googling found a few people with similar problems, but I haven't found any solutions yet.

Please, somebody, help me!

It has yet to freeze on the new account, which kind of has me relieved because that would indicate it's a software thing, not a hardware thing.

Also, just for kicks, I ran fsck but it didn't find anything wrong.

I'll try using my original account again and see if it still sleeps.

Update: Exactly a week later (Sunday night just as before) it started doing this again. It did it on my "new" account as well. It is now Thursday and it STILL DOES THIS. I can't get any of my video project done and this is really killing me. Are there any other suggestions please!?

Thank you. Do I have to have AppleCare to do that (because I don't)? How long did it take? Did you send it to Apple or some other certified repair place?

Sorry for the questions.