system not stable under stability test

[HipHoper]

I'm trying to build a spare computer system from my son's old P4 (775) - 3 GHZ computer so I can test RAM/Video cards/Power supply on it when needed.
The thing is that although my son used this computer for few months without any problem, I cant make the system pass some stress tests that i've tried on it (Furmark 1.9, Burnin test 6 Pro) without it to restart itself after few minutes (In Burnin 6 i use max settings and it restarts in less than a minute).
I've replaced the PSU, VGA,Ram and CPU to Celeron 2.66, But is still restarts under these stress tests loads......(Furmark gets 7200gs card to 85C from normal 66C during the test - it's a fanless version of the card)
I wonder if it might be the motherboard, Or I need to find another way to test this system, Because it feels to me that it's something about those test programs.....
It's Asus p5vd2-vm via chipset based motherboard....it's southbridge chipset is completely naked - no heatsink.....might be overheating......need to check it out...

Update : Added 60mm fan to the vga card and Furmark is running for 20 minutes now.....Also great temperatures (60C instead of 85C) Will test it over the next few hours.
Onboard card is to crippled in his feature to run with furmark, So could only test the onboard with the burnin test 6 program which I think has flaws or I set it to too high stress settings......

[MobilePCPhysician]

Are the cmos settings set to the lowest settings?

[Ferrit]

check the motherboard caps

[HipHoper]

Hi Guys,

Bios settings are not touched and set to defaults. Caps looks fine.
It has completed 8 hours with the furmark 1.9.0, So I guess it is stable now. I am planning to run the burnin test 6 with it's default settings (Not max settings as I tried before) And see how it behaves.

Thank you

[Niclo Iste]

The fanless NVidia cards have a tendency to crap a system out when overheated. If this card has done it too often it will need to be replaced as it would have been damaged. I've resorted to adding my own fans before running any fanless NVidia cards to prevent this.

[HipHoper]

Hi Niclo :O)

Hmmmmmm.....I usually dont sell high end systems, So must fanless cards works ok for me. I guess that fanless 8500/8600 carda and such might be problematic. You also don't always use cases with ultimate airflow.....The only time I had problem with fanless cards was with 8400gs card, But i think it's because I've connected only 20 pins of the power supply instead of 20+4.....