I had the same problem earlier this year, and I point out that we have the same computer model that is nearly the same age. That said, I tried resetting the PMU to no avail. Finally, I took my PowerBook to a technician, and it was determined that I required a new battery. Installing the new battery took place about two months ago, and since, I'm pleased to report, I've had no further trouble and only better performance from my new juice provider.