AC power adapter sensing an overload of power drain
Overheating processor due to a build of internal dust (clogged fan)
AC power adapter not able to sustain normal power needs (faulty power adapter)
Battery overheating, due to internal battery fault, or charging levels
Intermittent short circuit or open connection (check condition of power leads)
On older Laptops, the most common reason is poor cooling. Try going somewhere air-conditioned and see if that helps. Inspect the areas near the internal cooling fan; see if there is a build up of dust fibers. Easy way to check this; with the unit running, feel the flow of air from the cooling outlet(s) - is the airflow very low? If yes, and the unit is at normal operation temperature, this may indicate the internal cooling system is partially clogged.