I've had a couple of these come into my shop with board issues. They are always miserable hard to diagnose problems and I've yet to fix one yet. This sounds like how they start to fail. Then it will not power on and the keyboard light will flash every 5 seconds. I have no idea what is causing this on them. May be a RTC issue (The thing that puts all the "thinking" components on a timer so they can all talk to each other efficiently) . But as of right now the only solution i've found is a new motherboard.