Sad to hear that Naru, have you had BSOD when crashing?
can you tell me brand ant type for your mainboard, ram , gc and power supply
have you checked the power switch connections?
have you check your main disk? is it full or nearby? have you had a defrag (if HDD)
sounds weirds indeed...
what boo said is a good way of diagnostic, the question is have you got another power supply?
if not you can test yours in a simple way : unplug all , but a fan, take the 24 pin connector and shunt green cable with any black one in the vicinity : if fan turns (when power supply plugged) all is clear if not you might have some troubles with it.
see this : http://www.wikihow.com/Check-a-Power-Supply
depending on your PSU brand you may have lack in frequency stability too : a noname PSU getting old can become deficient even when it boots because voltages are not stable, the best thing is to swap it in this case, some very good antec psu are not that expensive, it's all depending on what you have to plug in