Z600 Spontaneous Reboots

[DoctorWizard]

My Z600 has started doing spontaneous and random reboots.  It might go a whole day without a reboot, or only 30 minutes.  It may do it while I'm running a bunch of stuff, or while it is idling with nothing open.  I have been doing various things to troubleshoot it and I am fairly confident it is a hardware problem, but which component?  So far I have:

• Checked the Windows Logs.  There are entries at bootup where it complains about the file system not being properly shut down, but no entries preceeding it with any sort of critical error.  And there are no crash dumps.
• Run SpeedFan and a couple other system monitoring tools.  None of the system temperatures ever goes above 73°C
• Run Memtest86 for 24 hours.  It never found any problems, but it did reboot a couple of times while running it (not at the same point each time) so that leads me to believe that it is not the O/S or Windows.
• Blew out what little dust was inside and re-seated all the connectors.

I think I have a spare video card around here somewhere, and I think I'll try that next, but my gut instinct is the power supply.  I can get a used one from eBay for around $40.  I'm on a limited income though and trying not to puchase anything I don't really need.

Any thoughts?

[Lil_Boy_Blue]

Hi

Perhaps test the Hard Disk, if it fails then all your data is possibly gone?

First:- Turn off the computer, press and hold the PWR button for 4 seconds.

Then - Leave only keyboard and mouse connected, disconnect all externally connected
devices such as Docking Station, USB storage devices, displays, and printers.
Unplug the AC adapter from the computer.

Next - Press and Hold the Power button for 15 seconds to Hard Reset.
Rinse and Repeat

Hope this is relevant...
AND - Power on your PC, press Esc (repeatedly), then F2 at the prompt.
Choose System Tests (Fast Test does a 4 minute hardware check).

[DoctorWizard]

Thanks for trying to help, but...

It's not a old-skool hard drive, it's a nearly new Samsung 970 EVO Pro NVMe SSD in a PCIe adapter.

The O/S and the data are all fine, nothing lost, no errors.

It spontaneously reboots even when operating from a flash drive running Memtest86 or Linux MInt.

The Z600 is a engineering workstation (desktop), not a laptop.  It has an internal and proprietary power supply, not an AC adapter.

BIOS has no built in system test.  But numerous 3rd-party test utilities find nothing wrong with memory, processor, cmos, motherboard chipset, video card, etc.

Have unplugged all USB devices except keyboard.  Tried a different keyboard and NO mouse.

On eBay I can get a used power supply with no warranty for $40 + shipping.  Yet a refurbed motherboard with a warranty is $35 with shipping.  Go figure.