Comp shuts off after a second after turning on. Help to find the cause?

There is an old comp, judging by the firmware motherboards - 2006, which is used as NAS storage. Characteristics at the end of the under spoiler.

Problem:
• When you turn the computer starts, begins to rotate the cooler, and after about a second after pressing power off. Sometimes manages to squeak built into the motherboard signal. Sometimes he can't manage. After that the power button no longer responds until you do not turn off the power supply button or from the outlet
• After the voodoo to disable components (see below) booting, and the bios displays an error message AI Overclocking failed... and you choose to load defaultnya settings or to enter and configure the bios. But the fact that I did not disperse, and the problem is evident even on default settings.

Playback issues:
• If after a standard shutdown from the system the computer idle all night, in the morning he can no longer turn on.
• If you turn in a few minutes after shutdown, the computer always starts.
• If nothing is disabled in the periphery, then the computer will not start, only helps disabling some components. In this way the contrast of this topic. There from a friend wih helped off the PSU for 20 seconds.

A temporary panacea:
As already said helps disabling some components. And always different:
• Sometimes the computer is got after the extraction of one strip of RAM. And sometimes only after removing both straps.
• Sometimes it helps to rearrange the RAM from one slot to another (in cases when there is only one strap)
• Sometimes it helps to disconnect the monitor from the VGA input of the computer
• Sometimes triggered if you eject the flash drive and keyboard from the USB ports
• Sometimes when you disconnect the LAN cable from the network card (network card fitted)

Localization issues:
• Replaced the thermal paste on the CPU and the North bridge — the problem persists.
• Obviously, if you disable the individual components and the problem persists - then blame not those specific components. T. O. RAM, HDD, keyboard, monitor, are likely not to blame.
• Visually, the motherboard is clean. Swollen capacitors on the motherboard, no signs of corrosion no. Sledov mechanical damage.

Not tried:
• Do not change the power supply since take it anywhere, even at the time, and buy a new one for the test - not reasonable. If you find out that he is guilty, probably will have to buy.
• Seems to be problems with the electricity network in the new house. The system unit is connected to the same outlet, the monitor, on the other. Directly. Without a network filter. It is not present. To buy makes no sense.
• Do not change the thermal grease on the South bridge. Her condition is not yet know open not in a hurry, as I read the reviews on this motherboard - some guys when removing the heatsink off the chip. So if you try to change, in the the very least.