It might be a heat issue. Check that the ventilation slots are clear of lint and other debris, you may need to open the laptop and clean debris out of the fan shroud and heatsink fins. While you're in there you might as well pull the heatsink from the CPU, clean off old thermal paste and apply new.
Before opening it up though you will probably want to monitor temps to see if that's the issue. Try downloading & burning a copy of Hiren's bootcd
for some diagnostics that will help you test and monitor temps. It's self-booting so no installation needed.