iMac 21.5" (mid 2010) randomly freezes since OSX 10.6

Hey, my name is Alberto, I'm Spanish, so I'll try to do my best writing in English.

I've been having some weird situations with my iMac since OSX 10.6. It happened like 6, 7 times every year since 2010, so It's not too annoying, but I want to ask for some help. The system just freezes and I can't barely do anything (I'll explain this later). Since I've discovered some patterns with this freezes, I'd want to ask you about this, and see if we can solve it, or determine if it's hardware-related or software-related.

First of all, system info (EtreChech output, got it after the freeze): http://pastebin.com/cShE0AnF

I'm actually using 10.9.1, hoping that this version finally solved my problem after one freeze on 10.9, but it didn't.

This freeze was on 03:47 am, so I copied the syslog output around that hour too (if you need more, just tell me): http://pastebin.com/AYrbTsrn

I've discovered that this only happens when I use Safari, and also, it only happens in this months (November and December I mean, it never happened on another month, but it could be a coincidence).

It usually happens when I navigate on heavy-content websites, like Facebook or Youtube (for Facebook, tons of images, and for Youtube, The video itself and the ammount of comments displayed), but It happened too when the screen was off, and Safari was opened in the background. I tried to wake up the screen, but it only showed a black background.

I use AdBlock Plus, and I have Adobe Flash too.

I always keep opened iTunes, Skype and Mail app, and in this last occassion, two Minecraft Servers and an Starbound server set up on Terminal App. I usually check the memory and CPU usage, and RAM has like 65% free memory, and CPU like 5% or less usage. This last time, obviously, RAM free memory was less (like 25%) but CPU usage was like 5% or less anyway (servers wasn't being used, no players on them I mean).

What happened this time:

Desktop froze. I was going to click on a link on Safari (well, this time I exited a website frame, a Facebook image frame), and it just did nothing. I tried to scroll, change the system volume via the keyboard, but nothing happened. I couldn't switch between desktops or fullscreen apps on my main screen, nor in my secondary monitor, and the pointer displayed the "pointing to" image (you know, the hand that appears when you hover over a link). I remark this, becouse in any case I saw the famous spinning "beach ball" and I've never seen it when this happened times ago. I think, though, that the pointer could be irrelevant, I think it only keeps frozen in the last state it was, but it's just my impression.

What I've discovered in another freezes:

Skype keeps working, I can keep talking with my friends If I already started the conversation, and It nevers shuts down, until I force-reboot the system, obviously.

If a song is playing on iTunes, it keeps playing, and even keeps playing the entire list of songs.

Another interesting thing is that, connecting to my iMac by ssh works, I can kill processes (like Skype, shutting down the ongoing call), get some outputs cat-ing files, and so. I tried killing Finder, Dock and SystemUIServer, thinking that the problem was with the Desktop Interface, but It didn't solve anything.

I've restored the system countless times, without backing up anything.

I hope I've explained this well, if not, tell me so.

If I should add some more info, just say.

I've been searching the web about this problem, but I didn't found anything, maybe I didn't search it properly, I don't know, but I've spent hours and hours looking for a solution. This makes me think it's hardware related, but I can't find any error, I ran the Apple Hardware Test software too, without any warning (as far I could see).

Thanks,

Alberto Salazar.