I am having a problem here which I don't know if it is normal or not

But when I end up forcing the system to shutdown by holding the power button in for 5 seconds it shuts down ok but starts up automatically after 2 seconds and shuts down after 2-3 seconds, then after that when I power up the system it boot's up fine, same happens when I plug in the power cable to the PSU, the systems starts up (fans run) for a few seconds then switches off, Dell said this issue is normal

Have already replaced the motherboard, PSU and have removed all the PCI cards.

Any idea's?

Cheers