|This is a problem lying within power distribution--or a heat problem, which can be repaired if you are comfortable opening up your computer's innards and looking over this small checklist.|
a. Check your internal vents, ducts, fans, and the thin processor heat sink. Buy a can of compressed air and carefully blow out any dust before you begin. Do not use a vacuum cleaner on a notebook! If the grease is crusted up on the processor heat sink, you need to carefully re-apply new grease.
b. Check your internal connections for loose components, and that your power adapter works properly. Is the power jack loose, or is the battery dead? Can the computer boot from the battery only and stay on, or does the problem occur only with the power adapter? Also, check to see if your adapter is getting overly hot (they normally get warm--we're talking hot), or if the power jack box on your motherboard is loose. This can cause issues.
If all these points fail, your motherboard may have an internal fracture from previous overheating or old age. This is usually a Dell(tm) and HP(tm) issue, but it could affect your PC as well, depending on how long you've used it.