In my case it was the mobo which was (at first i thought) incompatible with my CPU, soit needed a BIOS flash. After that, still the same though.
I ended up purchasing a new mobo & everything worked instantly.
Is your mobo mounted in the case or on a flat service? Do you wear any fitness or sports bands that could possibly cause static electricity?
It's mounted on a flat surface. The motherboard is on risers which rest on a piece of wood that is contained in a milk crate.
I'm careful about static electricity, I make sure to discharge any before working with the parts. Also it's fairly humid weather and the computer is in a room with a concrete floor, so the chance of damage from static electricity is very very minor.