Re: X48t-DQ6 strange issue
If you're ruled out the power supply, the next thing I'd look at on a board of this age are the capacitors in the VRM section of the motherboard. They're the little beer can shaped devices in the area around the CPU. Look for signs of internal overpressure (bowed out sides and tops...the tops should be flat to slightly concave) or leakage (brown goop or white crystalline "feathers") on the caps. Also look at the power MOSFET transistors for signs of serious overheating (they'll be heavily discolored and have a burned odor when they fail).
QX9770 @ 3.6Ghz (formerly E8500 @ 3.6Ghz)
4 X Corsair XMP2 DDR2 800 RAM
Sapphire HD5850 1GB
Ceton InfiniTV4 cablecard tuner
Apple 23 inch Cinema display + Dell SR2320L