My wife's iBook G4 1.42mhz has been having some strange problems on battery power. We can watch part of a DVD, look, see we're at 82 percent battery, watch for about the same period of time ... and thus should be around 60 percent battery at the worst.

Somewhere around there, the thing simply goes black and dead. No low-battery warning, no nothing. And the button on the battery itself still shows three of the four lights.

We can restart the laptop still on battery power, see the warning about the date reset. It might work for a couple minutes, then go black and die completely again.

Any advice? The battery's still showing a good charge, it should still have a good charge, there's just no way this thing's that far out of calibration. I did blow on the contacts just to make sure any loose gunk was removed, but I can't find a reference to anyone else having these sorts of problems.

Edit: And it's not a loose connection. I put the laptop on the bed and just watched a minute later as it died again.

Any chance there's a thermal problem, with the fan not running on battery power or something?