First go into System Profiler and check the health of your battery. (System Profiler is in Applications-> Utilities and can also be accessed via About your mac -> more info). Click on the Power tab in the left part and take a look at your Battery Information and the condition as well as cycle count.
I can't really think of anything else to do except to reset your SMC although I don't think it would really help. Wouldn't hurt to try though: support.apple.com/kb/HT1411
It could also be due to overheating, etc. as itghelp said. If nothing works you're in for a trip the the Apple Store.
EDIT: The battery is a possibly a reason for the random shutdowns (but by no means definitely the reason) but since the battery condition is poor I would consider replacing it. It shouldn't read anything other than Good or "normal." Depending on your full charge capacity and cycle count the apple store might replace it for free, but I don't know.
I would recommend replacing the battery (btw is it removeable?) and if you go to the apple store in the process also ask about the intermittent shutdown which may also be a result of something else. Keep your eyes open!