Most likely it is the power supply. If it is a Dell/HP/E-machine/Compaq, toss it if it is old (3+ years) If it is a "normal" PC with normal standard parts inside, replace the power supply with a decent one, and carry on. The on/off blip is very common for failure of the power supply. I see it all the time. Maybe 20% of the time it ends up being the motherboard, but far more often it is the power supply.

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