Did you change anything in your BIOS? are you overclocked? hyperthreading on or off? which type of graphics card? Did you calculate power consumption correct for the powersupply? are the cooling fans spinning? have you done a RAM check, could be bad ram - might have to take your ram out and put back in one at a time (with computer off obviously).
could be a lot of things. but, thinking about SL being graphics intensive -im wondering if your graphics card starts to suck more power than usual and your PSU isn’t keeping up.