Suggested steps to try:
1.Try bare minimum configuration only with hdd, processor and memory - disconnect the optical drive.
2.Try resetting CMOS.
3.If the unit has dual processor, remove the Riser card(having the second processor) and power the unit.
4.Try swapping power supply if one available.
5.Try swapping graphics card if one available.
6.If all has been tried the primary suspect will be system board/power supply.