Thank you , I missed that.
It is a very popular board and hard to believe there is anything wrong with it, But there is one clue. You said the PSU was replace.
Before you try to replace the mobo, Try this:
Prepare a boot floppy from another PC.
Remove the Video card and replace it with an older, low-POWER card.
Remove RAM sticks and replace with smaller, low power stick.
Remove the HDD. (Or just DISCONNECT poser and data cables.)
Remove the CD-ROM drive. (Or just disconnect poser and data cables.)
Remove all USB devices.

The idea here is to reduce all PSU demands.
Now see if you can boot the thing with a Floppy. We are not now using the Windows GUI OS, so that is out of the picture here. We are testing hardware.

If it works, something you removed is a power sucker and put the PSU was put on the edge of fold over. The floppy drive normally takes a surge of current WENT it turns on with a disk present and kicks the PSU into SELF protect shutdown if there already is a high current demand.

That is the best hope I can give you.