Computer help forum: Sony VAIO Laptop Won't Stop Locking Up

I'm contending with a Sony VAIO laptop. A VPCCW21FX to be specific. It's about two years old and has seen a lot of varying use - from just light word processing to moderate gaming, and everything in between.

Here's the specs:
quad-core 2.13GHz i3
8GB ram
Win7 64-bit
nvidia 310m graphics card
450 GB hard drive

Up until recently, this computer has been able to handle everything that I can throw at it. But within the last few months, it has started locking up without warning. Here's a general scenario:

I'll be doing something that shouldn't cause any problems, such as just surfing the web, when all of a sudden, I no longer have any control over the mouse or keyboard, everything just freezes. If I have anything making sound (music, video, whatever), the sound will just start repeating one note over and over and over in a strange type of stutter. It'll get stuck like this and the only thing it will respond to is holding the power button in (in roughly 0.5% of cases, it will get it's stuff back together after about 15 minutes, but most of the time the computer will just set there in an eternal state of locked up).

Sometimes this computer will go days without an episode, but the minute it has one, each subsequent one is closer and closer together (a second one almost always occurring less than 20 minutes after the first, and a third one shortly thereafter). It seems like about the only way to get it to reset is to leave it off for the night.

At first I thought this was a heat problem, so I took it apart and cleaned it out, fresh thermal paste, the whole deal. I set it back up on a nice cooling mat, but it still locked up shortly thereafter. Not to mention that I couldn't find a single part of the computer that was getting anything over a few degrees above room temperature.

Then I thought that maybe I had some sort of malicious software running around, so I ran every sort of scanner for malware, spyware, virus and anything else that I could get my hands on, and they all turned up nothing.

I've even ran this computer with absolutely everything shut off that wasn't either absolutely necessary for the computer to run or the one webpage that I was looking at, and it still had problems.

On top of all this, portions of the data on my hard drive have randomly disappeared, or certain folder names have been replaced by all boxes, that same box windows uses when it can't figure out what character belongs some where. The even stranger part is that all of the missing files are exclusively out of my music library. Roughly half of the information there is gone, and it's just random songs out of random folders.

I've scanned the hard drive itself for bad sectors and ran a few other tests on it; all of those say that the hard drive is fine.

I've completely run out of ideas on this one. If anyone's got any ideas on what's going on, I would appreciate it.