|How old is your motherboard?|
Is this an intermittent problem?
The symptoms suggest that it could be a motherboard that is sufferring from leaking / bulging capacitors. Motherboards that fail this way exhibit the kind of symptoms you are reporting.
Google for leaking / bulging capacitors and you will find more information on the subject.
You can visually check your motherboard in order to dismiss this as being the problem.
Similarly power supplies can also suffer similar problems. If your motherboard shows no visual signs of leaking / bulging capacitors then you may want to try a different PSU if you have access to one to discount it from being that.
☺ When everything else fails, read the instructions.