Sorry 'bout the delay. The worst of problems, or most annoying, is when something seemingly works... but stops working, to start working again... etc. I know very well, that statement doesn't help you.
Do you have your headphones connected to front or back? I need to restart, went somewhat the wrong path earlier.
Have you had this problem with the present motherboard since it was purchased new? I'm just mapping, others read this too, and we never knows who get's a light bulb burning.
But, it's possible that the socket is bad, and has been it all the time.