Slightly Frankensteined Z800 Shutting Off But Fans Still Run

[SeenRender]

I was setting up an old Z800 I got from a family member to be a server for various games. It was missing the GPU and HDDs, so I added a GT 640 I had lying around, and a random 250GB HDD. I was able to update the BIOS and Install Windows 10, but now every so often (15 min - 1 hr) the computer freezes up, and the fans spin very quickly. The computer needs to be shut down and rebooted, upon which it works fine for a little while.

It's got Dual Intel Xeon E5620's

20 GB of DDR3 across 10 sticks (2 were missing when I got it)

Aforementioned GT 640

Any idea what might be causing this issue? I'm fairly certain it isn't temperature related, as the CPU and GPU don't seem to be getting that hot, and the issue happens while idling.

Any help would be massively appreciated.

[DGroves]

after it's  on for approx. 5min, manually reboot enter the bios and check cpu temps/fan speeds and check system voltages for stability +5/12 volt lines should remain fairly stable and not have large changes

you may have to redo the thermal paste on the cpu(s), also check the heatsinks fans for proper operation the low power sinks use a 4 pin PWM 92mm fan high power sinks use a 5 pin power plug with a black wire connecting pins 1 and 5 together

z800 Bios List/features:

https://support.hp.com/us-en/document/c01718191

confirm proper ram layout for this system, ram is added in groups of THREE (not two/four) so your 10 sticks of ram is a improper configuration and may well be the cause of the random lockups

side cover should have sticker showing common ram setups

https://h20331.www2.hp.com/Hpsub/downloads/Mini_WP_Z800_memory.pdf

for 56xx xeons you need a rev 1.03 board earlier revisions will not usually reliably run 56xx cpu's

look at the 1 inch white label on the board located below the large black heatsink that covers the IOHs.  This "C/T label" has a barcode and several numbers.  If the board has an assembly number "AS#460838-001/002", the system will support only the "Nehalem" Intel 55xx Xeon processors.  If the board has an assembly number "AS#460383-003", the board will support both the Nehalem 55xx and also the "Westmere" 56xx Xeons. 

or, Go into the F10 BIOS BIOS settings, and look at the Boot Block Date in System Information.
     If the boot block date is 01/30/2009, the system only supports the Intel 55xx processors.
     If the boot block date is 11/10/2009, the system supports both Intel 55xx and 56xx processors.

case fans ramping up to full speed when starting system is usually due to a bad ambient temp sensor embedded in the power button cable, you can use a z620 pwr cable if the z800 cable is unavailable or replace the failed transistor in the cable

pm me for details on this if nessary

[SeenRender]

I used HWMonitor to check temps, and they seemed fine. I'll definitely check the voltage though. I do have an 003 board, and the CPU's were original as far as I know. I actually ordered a pair of slight better CPUs so I'll probably use those to see if it is indeed a CPU issue.

I think I'll also try pulling some of the RAM sticks, and leave 6 in the proper places.