Sounds like the whole system is locking up. This could be due to a GPU issue or it could be something else. You can check the Windows Event viewer to see if there are any events corresponding to the crashes but it can sometimes be cryptic. I recommend searching online to read how to use the Event Viewer.
For hardware validation, to begin I would remove any overclocks and run at stock. See if that makes any difference. Also, if this is a random issue but happens often enough to not be a burden you could try removing the GPU and using the iGPU instead. You won't be able to game but if you just want to leave the machine overnight to see if it crashes then that would be a simple test.
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