2.4 GHz (Late 2011) MBP Unibody 13" - Looooong pauses during normal use....no hard crashes. After checking the logs and noting some i/o errors I figured it was time to replace the hard drive. Again.
The fixit site directions on swapping the hard drive were excellent.
Turns out that: long pauses, and then severe difficulty in getting OS installed on replacement drive (both via time machine backup and Recovery tool), and hours of web researching, and a trip to the Genius Bar equals damaged hard drive cable.
(Not a bad drive, or corrupt files)
Also the re-install OS directions on the Fixit site did not include adding a recovery partition and installing a fresh OS via recovery tool. That would have been helpful. My first run used a cloned version of old drive. When I ran into problems I guessed that the drive errors had created problems on the new drive. Not so much.