Since you seem to have had problems from the very start, I would suggest you take your computer to service. It may have been a flaw in Windows installation, or a faulty component.
Overclocking... makes it a bit difficult, it may have invalidated your warranty. It may, even probably, have caused damage to your computer. If that's the case, the damage is done, and no new settings will help. Only hardware check can find out what the problem is.
It's a matter of conversation, and the seller's attitude towards customer. Not a clear case.
You could try how it runs with only one core, but that wouldn't solve everything.