This can be caused by several conditions. CPU Chips have a safety feature to shut down the computer when it reaches high (over heating) temperatures.
This can be caused by several conditions. Some of the causes are: insufficient amount of memory, a full hard drive, a corrupt system cache, a large browser cache.
Viruses come in various forms with a myriad of symptoms. A computer virus is a program designed to: harvest data, cause computers to run slow, destroy programs and / or data, etc.