Power Sags / Brownouts
You could be experienceing power sags or brownouts which cause the Power Supply to react as though the power is out. This will lead to damage to the power supply and possibly your motherboard in time. If any of your appliances trip your main circuit or if you go through light bulbs quite a lot this could be an indication of sags.
If this is the case and it isnt just a bad Power Supply, then I would recommend investing in a UPS or and SPS to protect your computer from damage.