If power supply swap has not fixed the issue - it is most likely system board.
However i would suggest to try few steps to check if that fixes the issue.
1.Try Bare Minimum configuration with only processor, hard drive and memory - disconnect the Optical drive.
2.Try removing the graphics card and check if that powers the unit up stable.
3.Try resetting cmos button.
If nothing helps please log a warranty call to replace system board in the unit.
