@geordienz ewwww. That really *****!
The "key" to diagnostics is to find a pattern. If you can figure out cause and effect, then it can usually be fixed. But things that happen "at random" are near impossible to fix.
One idea is to open the diagnostic message log before you go to bed, leave that on the screen. In the morning, if the machine is frozen, the last item listed might be the culprit or a clue to the culprit.
Also, you can open this log after recovering from a crash and read it back to the time of the crash and see what it says. You can tell when it crashed by the jump in the time-date stamps each line starts with.
The "bad" news is there is no guarantee that a message will be written before the machine locks up - its a race! Also, when reading the console log you may see a lot of scary looking messages that are actually harmless. Please don't panic. Google search anything that looks suspicious to see if you can find a website talking about it. Or post it here - in a fresh thread, not a reply to this message please.
Access the log by running the application "Console" in your Applications/Utilities folder.
You should not have to use FSCK on a disk formatted with Apple's Journaled system. I reccomend you do use Disk Utility to check your drive for errors and fix file permissions. The Apple page that describes how to do that - including the FSCK procedure you were trying is here: http://support.apple.com/kb/TS1417
I also reccomend you make a bootable OSX disk. That can be a DVD, USB key, or external drive (USB or FireWire). The procedure to make such things are here:
https://discussions.apple.com/thread/4183168
and here:
https://discussions.apple.com/thread/4663215
You can run Disk Utility from the "Recovery" partition - a good first step - by following the instructions here: http://support.apple.com/kb/HT4718
If Disk Utility detects a serious problem, it will direct you to make an external bootable OSX disk and re-run disk repair from there. If it thinks your disk is damaged, it will tell you to "copy off what you can" and start over. Not a good message!
If reviewing the console log does not reveal any likely suspects, then you have to rule out a hardware problem. The easiest way to do that is book a Genius bar appointment and let them boot your machine from their diagnostic disk. They will want to keep it overnight. If the machine crashes, its a hardware problem and their software will detect and identify it.
If it does not crash, then its almost certainly a software problem with something you installed. Figuring that out is usually fastest by doing a fresh Factory Install of OSX. Then import your user data, but not your programs or settings. Do those by hand, one at a time.
If you have a large external drive, this is actaully pretty easy. You can make the external a complete clone of your MAC, which gives you a bootable disk that has all your stuff! You can then wipe and re-install from that external disk. And make a disk image (dmg) file of your built-in disk BEFORE each major change. Because its very easy to RESTORE a disk from a DMG disk image file. So, when you find the machine acting up, you can roll it back to the last state and either skip what broke it, or figure out how to fix/avoid the problem. And so on.