I was having a very similar issue when I first built my computer. I narrowed the problem down to my wireless card, and a Google search confirmed my suspicions. I had been using the card in my old Vista x64 machine, and there were no issues, but there were issues in my new machine. Apparently the x64 driver for that specific card has issues with memory amounts >4GB, which causes lockups. Not all the time, but when doing very specific things (for me, it was uploading to certain websites). There wasn't an updated driver, so I stuck in a wireless USB dongle and haven't had an issue since. So I would suspect a network driver if I were you, since the lockups occur during network related tasks.