Shutting down at 20% power
Macbook Pro circa 2009, 2.26 Intel Core 2 Duo, OS 10.8.2.
Some weird things have been happening lately, but most obvious is this. At between 15-20% power, my machine shuts down. Or rather, goes into a power conservation hibernation. I plug in a power supply and after pressing the power button it wakes up. It's as if 20% actually means 0%.
Another anomoly has been the machine not waking from sleep. The keyboard lights are on, but the display does not wake, requiring a force restart. But I guess I'm more concerned about the first one at the moment.
This is unsettling, no idea how to troubleshoot. Thank you for any assistance.
OS X Mountain Lion (10.8.1)