PowerPC :: IBook G4 Keeps Shutting Down?

I had my iBook G4 a few years ago, and since I was about eleven when I had it, I'm not sure I always took the best care of it, but it's always worked pretty much fine up until about a week ago. It's started shutting down randomly. It also sometimes randomly goes to sleep in the middle of things. In particular, it doesn't seem to like starting up, or the internet, especially if I'm on youtube. I've tried a PMU reset, as it worked once before when my laptop wouldn't start up, but it hasn't worked properly. It's worked from anywhere from a minute from the time I start up again, to about a day or two at most, but it always seems to shut down again.

I really don't know what else to do with it... and I don't understand how computers work at all.