I'm having this frustrating issue and I can't seem to find out what's causing it. Unibody MacBook Pro, 13" Mid 2010 2.4GHz Core 2 Duo 4GB 1067 MHz DDR3 Nvidia GeForce 320M 256MB This started happening just recently on the latest version of Snow Leopard. I used my Lion USB boot stick to format the hard drive and basically do a full clean install of Lion. After the fresh install, the problem persisted. It would just hard freeze out of nowhere for no reason. It doesn't matter if windows are open or not. Sometimes it's 10 seconds after getting to the desktop, sometimes 2 minutes, 15 minutes, etc. It just happens. After freezing up, nothing can be done except a hard reset. Everything becomes unresponsive. I managed to update it to 10.7.3 and the other software updates as well. It still continues to happen. What could be the cause of this? What are other things I should try to remedy the problem? I've even tried resetting the PRAM/NVRAM - whatever that accomplishes, I'm not sure but I tried that a few times too. Thanks for the help!