hey SolidAge,
It sounds like the battery might have developed a memory problem. Where its think its fully depleated when its not and vice versa. I'd call apple and see if they can send you a replacement, but I bet that what it is.
I had my ibook do something similar. It would charge up to 72%, then stop. Apple had no trouble sending out a new battery, all i had to do was send the old one back to them. - Grady