This could be overheating, does the laptop get excessively hot? If so, open it up, and try and clean as much dirt/dust out of it as you can. Dust builds up inside of computers over time, and can clog vents, fans, and heatsinks inside of the computer, causing it to heat up. Clean out the fans and heatsinks, then try it out. If it is still acting up, the battery may be to blame. You can get a replacement battery on eBay for around $25.