Please help me with tough troubleshooting question

[ZodaEX]

Okay i've been repairing computers for years and have only seen this exact symptom twice and haven't been able to determine what causes this. My work PC will spontaneously lock up out of the blue at completely random times. I pretty much know it has to be the motherboard or the power supply, but this is a production machine and I didn't want to open it up and change around parts any more until I have a good lead as to what I should change. I was leaning more toward the motherboard, cause i've seen lots of PSU's go out and they never caused this symptom.

PS: I've run prime 95 for hours without any errors, and this hard drive is pretty much known to be in perfect order.

 

[Malkizadek]

Could be a software thing as well... But to test the hardware try replacing the RAM then PSU, as this is easier than replacing the MOBO. Try this out. If a no go, then try cleaning up the software, running a chkdsk, getting rid of spyware and such. Let us know…

 

[vick1000]

Could be the hard drive, especially if it's locking up then back to normal after a few seconds, or if opening apps (accessing the drive) takes longer. How do you know it's not the drive.

 

[ZodaEX]

The reason why I think it's not the software is because this a a clean install of xp, with all the very latest drivers loaded onto it from the vendor's websites. There is very minimal software loaded on this machine, just Nod32, adobe acrobat, firefox, and our point of sale software thats costs us 500 dollars each time we need it re-installed. The reason I think the hard drive's okay, is that this part was carried over from a previous system that had run flawlessly for years. The PSU and mobo are new, so I was most suspicious of them first. Can't I assume it's not the ram since I can run prime 95 stable for over 3 hours, and it dosent cause it to crash sooner or anything.

 

[Nenu]

Possible reasons for a lockup

heat
Clock or voltage settings are wrong (verify your memory and cpu voltages, report anything else that might be odd)
faulty component which could be anything in your system including USB devices
bad connection
short circuit
EM Radiation if your case is open in a noisy environment
Mains power issue (try using an UPS)
I'm not on top form today so there may be a few more to add to the list.

I have excluded software as it doesnt sound like that is an issue but a driver with a bug can cause lockups.
Try different drivers.

Also if the PC is networked it could be problem along that line.
For example if I turn off my switch or unplug the network cable on my PC while downloading, it will lock up. Odd but true and never happens unless I do the above.
Turning off my router on the other side of the switch does not crash the PC.
Try using the machine as a standalone PC not networked and see if it helps.

I'll return if I can think of more.