Our conclusion is that the power controller on the M/B is at fault. What do you think?
I am not one to draw hasty conclusions, especially when it is not me doing the testing. I am reluctant to make any comment on this, except to say "I would want to do much more testing before outlaying any of my money possibly unnecessarily on another mobo".
Did you test your PCI-e graphics card in another working machine?
Did you test your RAM in another working machine?
If both the above items checked out as faultless, the next question I would be asking is ...
Did you try with a working PCI graphics card?
What happens when you remove the graphics card altogether, and attempt to start your system?
What happens when you remove the RAM (all of it) and attempt to start your system?
Are any of the characteristics you see then, similar to the symptoms you are seeing when trying to start your system normally?
You haven't stated the origin of this computer, whether it was put together by you or otherwise .... but regardless of that .... and before wasting any more money on parts, I would suggest that you do the following (and here I am thinking of possible electrical shorting of the mobo with the box .... could be something as trivial as a loose screw somewhere it shouldn't be, or the mobo just too close to the box .... or something like that.)
Strip your system (everything) from the box and lay the mobo on a table (on a wooden board or cardboard box or some such non-conductive surface. Inspect the mobo carefully for any signs of damage such as cracking or scratching or bent contacts in the slots (PCI-e and RAM slots). Check that there is no sign of swelling or leaking of the capacitors. Connect up the PSU. Connect up the switches from the front of the box so that you can turn it on and off normally (easy to say, maybe not so easy for you to do, and you must be meticulous with recording where the various wires connect to the mobo). Fit your graphics card and ONE stick of RAM, and connect the monitor, mouse and keyboard. Try starting your system now. What happens? "Bench-testing" your system is the best way to go at this time, when things are getting "desperate". Try starting your system without keyboard attached, then without mouse attached.
The problem could indeed be with the mobo, but it is really pretty much "last" on my list of suspects, and really only diagnosable by ruling everything else out as a possibility (unless of course you have a "spare" mobo of the same or similar configuration into which you can connect all the other parts to test with). Again, all this must be done "out" of the box, to ensure there is no problem caused by fitting the system in the box itself. All this type of work is made much easier if you have an abundant supply of "bits and pieces". If you do not have this "luxury" then things are going to be more difficult for you and you may have to at some stage, outlay some money and hope for the best. If your bench-testing, or any of the other steps above, don't help to diagnose the problem, then perhaps you will have to "try a new mobo".
Also, test by replacing all connecting cables where possible.
Please let us know how things are progressing.
Best of luck.