Maybe your mobo died? Or is in the process of dying? I had that problem after I got a new mobo. Basically, after a a month or so my computer would either begin to start-up then turn off, though eventually it would start. After a while, it would just not turn on. Tried a bunch of things, figuring it was my case's power connects or my PSU but both proved to be working, in the end it was my mobo. I RMA'd it, got a new one and no problem since. I know the issue isn't the same, but perhaps it is the mobo? I'm no expert though. :/