I have a 15" 2.66 GHz Intel Core 2 Duo MacBook Pro with 4 GB RAM and a 320 GB HD.
My MacBook Pro was working fine until several months ago it started freezing randomly. It would freeze during normal use of the computer and would be completely unresponsive for a couple seconds to a couple minutes. Sometimes, I had to force restart. The frequency of the freezes increased, and one day, it just wouldn't boot to OS X. It was really strange, since I had Windows on the laptop using Bootcamp, and that would work. When I tried to restore using my Time Machine, it froze after an hour. I tried it a couple times using different versions and all versions froze. I had to a fresh install, and migrated my data using Migration Assistant. For a couple weeks, the computer ran fine. However, one random day, the original problem started up again. The frequency increased, and this time I took it to the Apple Store. They created a new user account, and it didn't seem to have the problem. I moved all my files from the old user to the new (copy & pasting). The problem disappeared, but reappeared like usual after a few weeks. I took it back to the Apple Store again, and they changed the hard drive. After I had moved my files over, I noticed the computer freezes randomly.
I thought maybe my iTunes Library or iPhoto Library were corrupted, so I moved them to the Desktop. iTunes and iPhoto created new folders in the respective Music and Pictures folder but the problem persisted. I have no idea what's causing the problem, and its EXTREMELY frustrating. Its very difficult to work on this computer.
I tried all the major debugging methods: repairing file permissions, repairing file permissions on the home folder, fsck, etc. Everything seems to be in proper order.