For a couple of months now I experienced sudden freezes. It turned out to be an overheating issue. I've moved into a new flat and here I use WiFi all the time (vs. the previous setup, where the Mac got network via UTP). Apparently the WiFi module (AirPort ... which is an Atheros AR5418 chip in my case) generates tons of extra heat (even with no traffic at all Shock) and the factory fan control does not account for it properly. Using smcFanControl I can increase the lower boundary of the fan speed and avoid overheating.