Troubleshooting display issues

[Chikara]

that's not a bad idea either, problem is I am not really confident to switch the graphics card myself, not really sure what I am doing. I had a look at it looks like it's fitted quite tightly in there and I am not confident I can do this myself.
When I first bought this system the shop assembled it for me. (That was a number of years back so it's well out of warranty now)

[1101]

Sounds like a mb fault to me .
Next time it happens, press the numlock or caps lock button on the kb, the kb lights should turn on/off to match. If not, the PC is froozen

Intermittent issues like this can be a b8gger to track down, could be PSU , RAM, vid card , motherboard
even a faulty HD can cause similar issues .
Unfortunately , you will need to start swapping out hardware to track down the cause .

Windows logs wont help , as it cant log anything if Win or the hardware hangs . You'll only see the restart logs

[chiefnz]

that's not a bad idea either, problem is I am not really confident to switch the graphics card myself, not really sure what I am doing. I had a look at it looks like it's fitted quite tightly in there and I am not confident I can do this myself.
When I first bought this system the shop assembled it for me. (That was a number of years back so it's well out of warranty now)

Removing the GPU is pretty straight forward.

1. Unplug your DP cable
2. Remove the expansion slot bracket screws holding the GPU in place
3. Refer to attached image....Push the retention clip at the end of the x16 slot down (this will lift the end of the GPU out of the slot slightly
4. Holding both ends of the GPU, slowly lift the card out with a moderate amount of upward force
5. To re-install just do the reverse of the above

[CliveM]

Removing the GPU is pretty straight forward.

1. Unplug your DP cable
2. Remove the expansion slot bracket screws holding the GPU in place
3. Refer to attached image....Push the retention clip at the end of the x16 slot down (this will lift the end of the GPU out of the slot slightly
4. Holding both ends of the GPU, slowly lift the card out with a moderate amount of upward force
5. To re-install just do the reverse of the above

Switch all power to the PC off first

[dugimodo]

Might be worth disabling all the sleep options in power management so the PC will never go to sleep, just in case that's somehow what's happening.

And yes a different cable is worth trying, especially a different kind of cable. I have a monitor that blanks randomly for less than a second if I use display port on it but it works perfectly via HDMI. Pretty sure it's the cheap cable causing that.
So if you have the option to try a different type of port as a test give that a try.

[Chikara]

Hi, yes all the power options have been set to not sleep /standby for a while, so I'm pretty confident it's not that.
I have today managed to get hold of a second monitor, so I have connected one via DP (using a new cable, not the old one) and the other via HDMI.

Regarding the suggestions above to remove the GPU, I also have a liquid cooling setup in there. Which has reduced the space a lot and I think will make it more complicated to remove the GPU. I'm not even sure I can get access the the GPU without removing the liquid cooling.

I will wait until the next freeze and check some of the things already suggested (eg LAN lights still working or not), and then I will report back here. It can often go many weeks in between this happening, so I will wait until it does, and let you know. Thanks for the suggestions so far.