Time for some [H] advise for troubleshooting...

Alright, here's the scenario - my family room (almost) 4 year old build is having intermittent freezing issues. No BSOD, nothing showing up in the event viewer except the standard improper restart message from having to hard reboot. Sometimes it can run 24 hours without a problem while being used by my daughter playing Minecraft and Youtube videos, other times it will freeze up 3 times in an hour with little to no load. Computer is as follows:

• q9550 @ stock 2.83 Ghz (due to constant freezing/reboots)
• Hyper 212 cooler
• Asus P5N-D motherboard
• Patriot 2x1GB PC2-6400 RAM (was 4, see below)
• eVGA 9600GSO 384MB video
• Corsair TX750 PSU
• Samsung 840 SSD
• 3 various SATA HDD
• SATA DVD on PCI add in card
• Windows 8 install (Technet test system, who better to test Win 8 than a 9 year old?)

Other important info:

• CPU, mobo, memory & DVD from original build in late 2009.
• HDD's all 2-3 years old. All pass a chkdsk /F with no errors found.
• PSU & SSD bought new on Black Friday sales last year.
• Coretemp = 35-40*C idle, never above 55*C under full load (so temp should not be an issue)
• Also have a folding/DCing q6600 Win XP machine available to swap parts with for testing purposes.

So far, I've done the following:

• Pulled all the RAM and swapped every stick out with my other box, and all have (individually & in dual channel pairs) passed memtest86 for 24 hours.
• Tried putting 2x2GB GSkill PC-8000 RAM in box, moved down from 4x1GB to 2x1GB, and tried all 4 DIMM slots in various combos, still freezing. Safe to say, RAM is not the problem.
• Completely uninstalled the nVidia drivers and reinstalled the latest non-Beta drivers, no change.
• Last, removed all other SATA HDDs and the add-in SATA card running the DVD - froze up less than 24 hours later with only the SSD plugged in, then again within 3 hours.

Now, I'm trying to decide what to test next. Have had suggestions on a CPU swap, possible SSD problem and PSU issue. Therefore, I'm taking suggestions on where all of you think I should go next, as shown in the poll. Thinking of doing a fresh Win 7 or 8 install to a different HDD to check for a corrupted install (all data backed up and full disk image already made) or faulty SSD next, then trying to swap CPUs, then PSUs. If all three of those check out, I guess I'm going to call the motherboard bad and go from there. Anything I'm missing or haven't considered?

Suggestions NOT helpful: buy a newer system (going to when Black Friday ads come around, hopefully), downgrade Win 8 to Win 7 (Win 8 ran fine for 4-5 months before issue began), etc...

Please give me your advice, [H] - this part time problem solving is getting tedious!

Sounds like you've done some good troubleshooting already. I see you took out all of the SATA devices except one, but did you also try different SATA cables and ports? I just recently had my HTPC "die" where it wouldn't boot up at all, but found the cable had gone bad. Replaced it and I'm back up and running fine. Also, if you're wondering if it's the OS install, do you have another drive you can toss the OS on from scratch and toss in there by itself for testing? Possibly even one of the drives you'd used for secondary storage? That might tell you if something is corrupt, or even if the drive has some kind of an issue. Lastly, do you have SMART enabled for your drives? If not, enable it and after a while, check the logs to see if anything shows up.

To me it really sounds like either a cpu or psu problem. I've had the same thing happen on an older machine it would randomly reboot.

finally after pretty much building a new cpu minus mobo and processor I swapped a new cpu in and it ran fine. I would recommend doing the cpu/psu and I would do which ever you know you have a good spare for first. After all there is no reason to buy one component if you haven't swapped in as many spare parts you have around the house first.

swap the sata cable first. I've seen them go bad. it's so easy to swap and I'm sure you have spares.