Difficuilt problem. It is never goot when the problem occures after a time span. Sounds for me like there could be a themperature problem with one component
- I’d check the CPU fan / temperatures and maybe the thermal paste / or thermal pad on the CPU
- Next I’d change the video board. I had system freezes with a damaged video board. The system was crashed, but the last screen was still visible. Even the mouse cursor was frozen.
- At least I’d check the RAM. It is always a good canditate in case of freezes
- Maybe check the CPU itself
If you overclocked / over or under voltaged your system set the default values.
For the time span you try to find the problem unswitch and unplug all devices you do not need (additional HDDs, additional ODDs, Soundcard, …). It is easier to find the problem if you start with only a few components.
I.e. if you switch off all your additional HDDs, all ODDs, Soundcard and the problem is still there, than you know this components does not make problems. If the problem is gone you know one of this components makes the problem.
To find out the problem if you have only a HDD, Motherboard+CPU+RAM and Video board running you need tho exchange this components. There is no other easy way to find the problem… For this reason it is always good to have a few spare parts