Repeated freeze crashes (no BSOD)

Cross-posted at Question - Windows 10 machine crashes by freezing--hardware or software? | Tom's Hardware Forum

CPU: Intel Core i5-4670K
Motherboard: ASRock Z97 Extreme6
Ram: 2 x 8GB DDR3, Ballistix
SSD/HDD: SanDisk 1TB SSD
GPU: NVIDIA GeForce GTX 1650, however, currently removed for troubleshooting. System is running on on-board Intel HD Graphics 4600
PSU: 650w, cannot see manufacturer name
OS: Windows Home 1903 (x64) Version 1903 (build 18362.1016)

PSU originally installed November 2008
Memory and CPU originally installed 11/2013
Motherboard installed 10/2016

Updated: I have been running MemTest today. The computer locked up on pass 1/4 during the Modulo 20 test. The pass was 75% complete and the test was 54% complete. Then I rebooted and it froze again on pass 2/4, also during the Modulo 20 test. The pass was 75% complete and the test was 82% complete. It did not report any errors, the computer just froze. Does this suggest memory failure?

I am getting unpredictable crashes where the computer becomes unresponsive and the image freezes on the screen. I find this failure mode baffling. I don't understand how you can have a crash yet the graphics keep feeding the image to the monitors.

When I say unresponsive, I don't mean the machine is too busy to respond to interrupts, I mean it's dead. The NUMLOCK LED on the keyboard doesn't change when clicking the key, no mouse cursor movement, the screen image is frozen. The board still responds to holding down the power button to power down, and then I power it back up.

This build is maybe 3-4 years old and this problem started in March.

I have disconnected USB devices, the second monitor, removed the GPU. It still crashes. I run Task Manager, HWiNFO, and Resource Monitor. Occasionally the crash will occur when the CPU is 100% or close to it. Occasionally the cores will run hot near 100°C, but many crashes occur when the CPU is running under 50% with cool cores.

The crashes most frequently occur if I am using a videoconferencing app for work, either UberConference, Microsoft Teams, or sometimes Slack (although I've never had a crash using Zoom). But sometimes I can use these for hours with no problem. Some crashes have occurred overnight when I am not there and not running any applications. Two nights in a row it seemed to be when Malwarebytes was running a scan.

I can't find the right conditions to forcibly reproduce a crash.

What should I be looking at? Is this likely a hardware problem (bad board, bad memory) or more likely a software problem, or some combination where the software does something to the hardware that it's not supposed to do?

I have had one recommendation to do a clean reinstall of Windows but I don't want to go through all of that and then find out I have a bad chunk of hardware somewhere.

Windows 10 Home (x64) Version 1903 (build 18362.1016)
Board: ASRock Z97 Extreme6
I removed the GPU for testing and am running on the on-board Intel(R) HD Graphics 4600 [Display adapter]
2 x 8GB of DDR3

Can you check if your computer works correctly with different operating system like Hiren's BootCD.

Download | Hiren's BootCD PE

FreeBooter said:

Can you check if your computer works correctly with different operating system like Hiren's BootCD.

Download | Hiren's BootCD PE

I have already booted it to a USB with MemTest and it crashed twice while the test was running.

Do you have more then one RAM stick connected if yes then test the RAM sticks one at a time.

Does Reliability History have entries covering the crashes? The ---view tech details--- can give information in lines 1 and 2,which can be used to search for answers,e.g. microsoft site.
Are you able to run---sfc /scannow--- and---dism /online /cleanup-image /restorehealth---to help fix any file errors and hopefully eliminate Win 10 as a source of the problem?
It is easier and cheaper to exclude software first,before turning to hardware,which often requires replacement to positively proove fault or not.