My narcoleptic iBook

Been having a few problems lately with my iBook, which is now over a year old so out of warranty. Why is it that these things always happen just out of warranty? Anyway, I was very alarmed last week when my Mac suddenly started falling asleep without my shutting it down. It was plugged in and the socket light said it was charging, so I couldn't figure out what the problem was. It would fall asleep on me, then I would wake it up, only for it to fall asleep again.
I consulted my Mac guru friends (thanks guys!) and concluded that maybe the power management unit (PMU) needed a restart, so I followed Apples instructions for a PMU restart and things seemed to get better.
A couple of days later, I'm sitting here typing away, as is my wont, when the screen flickers and then the laptop goes to sleep again. Dammit, I think. That's very annoying. But this time, I actually saw it do it. I'm typing, then it just flickers and goes black. No message, no warning, no nothing. The socket light is orange, saying that it's charging. But when I think about it, it's been plugged in all day and the light was green. Certainly, it's been plugged in long enough that it should be green, not orange.
I check the battery, and the battery is flat as pancake. That'll be why it keeps going to sleep then – not enough power to keep awake. I change over the power brick to my spare, and bingo, it starts to charge properly. I figure that the power pack is at fault, perhaps because the outside ring is c-shaped instead of o-shaped, meaning that it's not touching what it should be touching and is not completing the circuit.
So I used my newer power pack and all seems well.
But today, same problem. iBook has been plugged in all night, with the newer power brick, but when I wake up and turn it on, it lasts a few minutes before going to sleep. I check the battery, and it's down to one flashing light. Oh dear.
So, question is, is it the power brick, or is it the socket? Or is it, perhaps, the PMU or the battery? Guess I'll have to take it to a Genius Bar to find out for sure. Maybe I'll waste hours in the San Francisco store, rather than the UK one because no way am I going have time to get this done before I leave. But if you have any thoughts, please comment. 🙂
UPDATE (1 Sept 06): Ok, so I took the iBook to the Apple Store and to an Apple reseller, and they both sucked air through their teeth in a very disconcerting manner and said how expensive it was going to be to fix. It seems that the problem is most likely to be with the connection between the socket and the motherboard, and fixing it will require a whole new mobo. Which will not be cheap. Now, I haven't had this confirmed because that would mean being without my iBook potentially for 10 days or more, which is just not possible.
My computer is essential to my life – without it, I cannot earn money, pay rent, buy food. It's no exaggeration to say that losing my computer (nevermind the data) would be a disaster. So there is only one thing I can do – buy a new one. I will then back-up all my data on to DVD, and see if I can't then get a proper diagnosis, and prognosis, for the iBook. I'll update here when I do that, in case it turns out to be useful info for anyone else having the same problem.
UPDATE: Today (12 Nov 06) Kevin and I replaced the DC-in board on the iBook, using one we bought for $30 and the iFixit Guides to tell us what to do. Works perfectly now.