my daughter bought a used ibook g4 15" and it worked for several months before developing a maddening habit of shutting down without warning. I've read other threads about this with ibooks but they say it happens only when the battery is operating and doesn't happen when plugged in. My daughter's does it either way and will only stay on for ten or so minutes most times before shutting down. It makes the machine unusable. We've checked the power supply and the power input board. Is this a logic board issue? She paid $450 for the computer but the prices have come down and it might not be worth fixing. I can solder and can get into the logic board if someone can tell me what to do. Thanks.