Two immediate causes for this are "bad caps" or a faulty PSU that may contain bad caps or just itself going bad. If it re-powers back-ON, then that suggests you have the reboot from lost power-OFF when configured in the bios.
bad caps - Due to aging or bad manufacture capacitors will start to deteriorate and that's not unusual for electronics. However, these are found on the mtrbd. and just everywhere else. Google "bad caps" for examples and what to llook for.
PSU - It has reached a point where to wear&tear or exceeded power demands have caused an unstable operation
Replace or repair, but if PSU, that's easy replace with a similar or better capable PSU suited for your needs. As for bad caps, while that can be repaired usually the cost is far better spend on replacing unless your're capable of such repairs.
tada -----Willy 
I am currently running an LG Pentium desktop PC with Windows XP and have had it for 5 years. Recently it has started switching itself off while in use, for no apparent reason and then switching itself back on again automatically. Sometimes it does this 4 or 5 times in a row and sometimes not at all for days at a time.
Any advice as to why this is happening and how I can fix it? And yes, have thought about slinging it out and buying a new one but want to avoid the hassle if there is an answer to fixing this one.Thanks.
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