I dont think that there is anyway to determine whether it is the CPU or the mobo at fault, if either but it does sound like a faulty CPU at this stage.
Something is putting a higher load on the PSU than it should by the looks of it.
You said that its a secondhand board, does that mean that you have installed it in your own case?
If so you havent missed the spacers out and shorted the board to the case ?
If not can you try that CPU in another PC to save you buying one to try in yours
![Huh? ???](http://www.pc-pals.com/smf/Smileys/Smileys/huh.gif)