The hard disk in my Mid-2009 MacBook Pro crashes intermittently.
In Activity Monitor, the disk activity flatlines and no data is written or read for a period of time, usually between 15 seconds and one minute.
The beachball appears and the computer is fairly unresponsive.
The active/current app will often appear as "Not responding" in the process list.
I have not been able to accurately induce it, and seems to occur fairly random although it does happen more frequently during periods of sustained disk activity. Watching YouTube videos, is almost certainly guaranteed to induce a freeze, as does Photo Stream syncing.
This started happening when I changed over to an SSD late last year, and after finding other people with my model of laptop, reporting the exact same issues I was convinced that an EFI update to the mid-2009 MBPs was the culprit. After Mountain Lion was released, I swapped to a new (not the original) HDD for a fresh install OS X and much to my surprise, it is still happening. I think it might even be worse (the duration feels greater).
I have absolutely no idea what is causing this or even how to troubleshoot it any further.