If it takes 4+ passes to find any errors, it's highly unlikely to be a memory fault.

PSU OK? I've seen PSUs with failing caps produce massive amounts of noise/ripple which can cause this.

I don't have a solution for you, but I do have sympathy.

it helps a lot if you have a donor machine to borrow from. My machine started BSoD'ing after a power cut (while running). Sometimes it would go 15 minutes, sometimes it would fault near end of boot. New Ram, Power Supply, HDD and finally MoBo (one at a time) didn't help. Turned out it was the CPU. I now have 1.8 machines 😒

Just also be aware that MemTest86++ V5 is now pretty old and isn't being maintained. There are known bugs in Test #7 that can give false errors.

People wrote patches to fix the bug but they were never included into the public release of MemTest86++

The MemTest86++ support forum is also dead, but here is a screen shot from 2019, referring to the bug found in 2017.

Thanks, I'm using the version from April last year.

But helpful to know there may be an issue with the test that was constantly failing.

Will run windows memory diagnostic tool next, currently have a 1 hour OCCT running to put max load (CPU and GPU) on the system to see if PSU can power it for the hour at max loads.

I'll the the windows memory diagnostic tool, then do some Furmark testing as well seeing how that holds up as well.

I've also swapped out the GFX card to a spare I had to see if that stops the random freezing/pausing as well.

If all three test work ok, maybe it was a faulty GFX card, will give the PC back to the friends if those all work fine so they can use it for a while, noting if they have any issues etc while using it normally.

andrewNZ: Have you tried the windows memory diagnostic tool?

OCCT Ran for a full hour with no issues (i.e. crash/reboot)

Windows Memory Extended Test running now.

HCI Memtest is excellent for testing RAM. I paid the trivial sum for Deluxe to make it easier to use on a bootable disk. It found a problem that Memtest didn't find that was corrupting files on my disk as they passed through memory.

HCI memtest free runs in windows and tests 2GB RAM max, so you run multiple instances. That found problems for me in the past, but it can't test the memory in use. Paying US$12 you get the bootable version that can test virtually all the system memory.

SomeoneSomewhere:

 

If it takes 4+ passes to find any errors, it's highly unlikely to be a memory fault.

 

not quite true.
A single fail would point to a hardware issue, could be RAM , or even just the RAM slot
Failing RAM doesnt adhere to our expections when running tests :-)
Even new warranty replacement RAM can be faulty, (cough cough ADATA) , change of brand & all was good .

Installing new RAM & getting the same issue points more to a motherboard issue though.
The motherboard is about 5 -7 years old ? . Chances are its dying .

If it's nvidia driver related, have you cleared those and reinstalled.
Nvidia run their GeForce Experience software that updates this quite easily.
Also consider a GPU stress test to put the card through it's paces.

Or as others have said, it may just be time for a new PC.