Unplug everything.
The ram, your video card(s), your hard drives, the PSU, all of it. Then pull the battery for at least 10 seconds. Put back only what is absolutly needed to get to bios screen. Power that sucker up and see what it does.
If that doesn't work, then all I can think of is that there may be a massive heat transfer failure between the heat sink unit and the CPU. What thermal compound was used?