Let's get some deeper details. Have your run the onboard diagnostics (depending on the running OS) or the CD/USB version?
Also let's install this gem of an app so we can see how your battery is doing [https://www.coconut-flavour.com/coconutbattery/|CoconutBattery] take a snapshot of the apps main window and post it here for us to see [guide|21499] Given your systems age its likely in need of a new battery and this could be the cause of the running how as the charging logic and SMC maybe running hard.
I would also look at your drive how full is it, maybe you should backup your stuff using TimeMachine, erase and reformat your drive and reinstall the OS, Lion (OS-X 10.7.x).