Crashing on warm days - I'm stumped

[TuxDK]

As the title says, my PC crashes constantly when the ambient temperature gets over 24'ish degrees or so. I don't have aircondition so the room temperature follows the outside temperature somewhat on warm days. Sometimes it will crash before it's done booting windows, but it takes 15-40 minutes for the first crash of the day, then after that it's usually 5-10 minutes. When assembled in the case a lower ambient temperature is needed before it crashes, so these days it's always assembled on my desk with good airflow underneath the mobo.

System:
mobo: P5Q-PRO
CPU: Q8200 (heatsink: coolermaster hyper 212 plus)
GFX: 4870
Ram: 2x2gb Kingston HyperX
PSU: Antex Fusion 550
Harddrives: OS: 200gb seagate (7200.7) / storage: 1tb WD "green"

What I have done so far:
Changed the stock northbridge AND southbridge heatsinks for thermalright hr-05-sli. These were running pretty hot but now they are cool to the touch (both the coolers and on the bottom of the mobo where the chip is).
The stock northbridge heatsink is connected to the mosfet heatsink, so I just bent the heat pipe carefully, so the old northbridge heatsink is held in the air above the mosfet heatsink.

Exchanged the GFX card for a 4770 from my secondary PC.

Exchanged the RAM for corsair xms2 pc6400 from my secondary PC.

Used an external fan on both the RAM and the harddrives to check that they weren't the issue.

None of those things solved the issue, and my temps are something like this:

GFX: 50-52 (under load, with 50% fan speed)
CPU: 50-55 (under load)
Harddrives: 30-33 (using speedfan)
The PSU seems REALLY good and feeling it's case and the air coming out of it after it's been running, it seems completely cool, but I haven't tested with another PSU yet.

As far as the mosfets go, I doubt they are the issue since I can remove the mosfet heatsink completely and they don't get insanely hot or cause the pc to crash faster. I'm really stumped, everything I can think of seems pretty much cool to the touch and I'm wondering what I should try next, should I exchange the CPU? Maybe its faulty? Should I try cutting the seagate harddrive out of the equation since it's somewhat old? Everything works fine on colder days, so I'm assuming it must be a temperature problem - but I just can't seem to find anything running hot!

 

[Modelworks]

What error is it giving when it crashes ?

 

[TuxDK]

I'm not getting any error, it just shuts down. And usually if I try to power it up again right away, the indicator LED on the mobo just blinks a few times and the fans power up for half a second before stopping again, after waiting a few minutes it will boot again.

 

[Modelworks]

I'm not getting any error, it just shuts down. And usually if I try to power it up again right away, the indicator LED on the mobo just blinks a few times and the fans power up for half a second before stopping again, after waiting a few minutes it will boot again.

Sounds like an intermittent connection. They can be tricky to find. Usually a solder connection is fine when cold but as it heats up it expands and cracks breaking the connection.
Start by disconnecting everything except motherboard .
Remove every card from the slots and all drive connectors to it.

Now let it run in a hot room with just the motherboard , cpu , memory, heatsink and fan. Use 1 stick of memory . You will not have a video card connected so it would help if you have a speaker attached to hear the beeps to let you know it has reached the bios stage.

If it can run like that then you can move on to adding the second stick of memory and testing it. Add the video card and test again. Keep adding 1 component at a time until it starts powering off.

 

[jdjbuffalo]

I agree with Modelworks. It sounds like a solder issue, probably on the motherboard. I have also seen this kind of behavior with the power supply. I am leaning towards these two as being the most likely.

Do the troubleshooting steps that Modelworks has outlined and that will help isolate the issue.