PSU PSU PSU.......get out your multimeter and back test every power connection to that mobo: 12v rail; 20/24 pin and the grounds. If gounds show leakage (more that .0010vdc) then your mobo is toast. Probably the PSU and I'll bet you see that the 12v rail is fluctuating heavily under full load. Then replace the power supply. However, you can just swap out the PSU with another for test purposes and see if it boots and runs noramlly. How old is that 600 st (which a really hefty PSU) PSU?
Noli Nothis Permittere Te Terere!!
EVGA 680i, Dual EVGA 8800GT'S (650/950), 4 gigs OCZ DDR2 1066, Intel Q9450 at 3.8 Ghz, Koolance Water Cooled, Dual Plextor16x DL DVD+/-RW. Dual WD 250gb 16mb Cache Sata 3.0. 3DMark06 score = 19,168.