Posted by Sinister Crayon definitely a problem in the motherboard. Possibly caused by PSU.
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Does it die completely, no power after it does it's dance?
Always observe ESD safe handling protocols when working inside a computer; the components are very sensitive to static damage. Failures don't always occur instantly, sometimes it takes days, even years to manifest itself, and become evident damage was done.
Check mobo mounts; make sure mobo is centered and nothing is shorting out to the chassis, look hard! Examine all wiring ans connectors from PSU.
House wiring; solid earth ground and good neutral return at wall socket. Screwed up neutral returns can cause weird problems.
Is the CPU heatsink working? Is the CPU going into thermal shutdown, maybe?
Verify heatsink integrity for CPU. Heatpipes fail.
Defective or shorted out fan, including the CPU fan and any mobo chipset fans; try disconnecting, but don't forget about them, or run very long!
Mobo, or PSU. Swap in a know good PSU; failures are common, and the rail voltages may measure out correctly and seem ok, but not be.
If you can run it on one Ram stick, do so. Trust nothing. Swap out with a known good copy, or test on another machine the vid card, Ram, any other PCI cards pull for now, and drives (try disconnecting DVD/CD drive).
Sometimes just disconnecting contacts on a Ram stick, vid card, power connection, etc will clean the contact by reseating it, and cure the problem. Never touch any card contacts with your fingers (body oils can foul them), and clean with isopropyl alcohol if you do need to clean them.
Have you tried clearing the Cmos by pulling the battery, your settings may have become corrupted.
Some boards will behave like this if they crash from the bios settings being corrupted, a hard reset and reentering the settings will cure this. The bios chip maybe fried; does it have a removable bios chip? Some Asus boards do, and it's not too hard to snap out and replace. Otherwise the mobo needs to be RMAed.
Call the mobo manufacturer if all fails, some can be very helpful.
edit - clarification