It is odd.
Older hardware...symptoms arise after being on for a time...
...could be hardware affected by heat.
Can you shut down, open the PC, clean it as best as possible (looking to remove accumulated dust, etc), and reseat cables, cards and memory? Not sure of your comfort level with something like this, but it would help to minimize (but, not remove, unfortunately) hardware as a culprit.
After that, I would suggest making sure you're fully updated (possibly against another repository).
This is full-on troubleshooting, so you'll have to decide how much time you want to put into this (versus backing up all your data, checking the HW and re-installing and restoring).
Hopefully, if anyone else has any suggestions, they will voice them as well.