Auto power down on battery problem

I consider myself computer savvy but this problem really has me stumped, hopefully someone can help me. I have a Thinkpad X61 that is about 2.5 years old. A few months ago, I noticed that whenever I use it on battery power and am somewhere between 40%-25% charge left, it will suddenly and very quickly indicate that the battery is low and then immediately hibernate. The battery will go down to 10% - low battery indicator, then 5% - critical and thus hibernate. If I turn it back on it will operate as normal but then repeat the behavior shortly after. I have tried 2 different batteries with no change. The two batteries were purchased at the same time and was part of a recall so I thought for sure the problem was with the batteries. However, I just received a new battery and on the first charge, at 40% the problem still exists. My computer was also recently formatted from vista to windows 7, so I know its not an operating system issue. I am at a loss as to what can be causing this. Any suggestions would be appreciated.