the capacitors on the motherboard have a tendicy to go bad over time. When the go bad they start to bulge. If they vent you'll see some brown crystalized ooze on the top. In order to check them you should gut the pc or take out the motherboard and look it over really good in a well lit room. If you arent confortable replacing them your self there may be a small electronics shop in your area that may be able to help.
Your symptoms are almost the same as what was happening with my compaq, it was fine for internet, but after a few minutes of WoW it would hard lock. I blamed it on my wife since I didnt play at the time. When I was putting it back together for her so she would get off my new pc that replaced it, I noticed the bulging cap and it all fell into place. I replaced it with a new one and not a problem since.
If you take it somewhere take it to a mom and pop shop, dont take it to radioshack or bestbuy. Otherwise you may want to start looking at a new board.