Hey,
I have a pretty neat rig set up (i7 2600-K, 8 GB of RAM, Radeon HD 6970) with adequate cooling. Yet, randomly, every now and then playing the game causes the PC to just hang. I've played games that are actually resource-intensive (Crysis 2 or The Witcher 2 comes to mind), yet none of these cause such symptoms. My OS is Windows 7 64-bit Ultimate SP1, Catalyst drivers are up to date, even installed latest hotfixes out of desperation.
When the lockup occurs, the sound buffer content repeats itself over and over and neither Numlock nor the mouse cursor responds. Strangely enough, the cursor becomes double its size during the hang. I don't get any crashdumps, nor is there a black screen.
Oh, and I'm using latest Ender's client.