Well I am still unclear. I admit that it still does sound like a motherboard issue at first glance, but it might not be. I would be much more thorough in playing MP3s or videos and checking out another game. Check to see if there are any conflicts in the system manager/devices in the control panel.
When diagnosing a problem you have to be very detailed, precise, and methodic. It's kinda hard guessing over the internet with such a 'general' description of a problem.