TiBook dies when put to sleep

[Jay Boshara]

I am new to this list, so please forgive me if this issue has been dealt
with before.

My wife's 800 MHz 15" TiBook (running on 10.2.3) has a weird problem.  When
she closes the screen and thus puts it to sleep, sometimes the computer will
be off when she raises the lid again -- like it just shut down (quite
unceremoniously) when she closed the lid.  This seems to happen only when
she is running on battery (and always when the battery was nearly fully
charged), but it doesn¹t happen very often.  Any ideas why?