Had similar problems a while ago with a 3,1
RAM was good - checked everything I could...... days went by.
Video card was the culprit ... very random and intermittent.
Had to buy a new GTX120 at the time IIRC
Can't remember what the original card was but it was "known" to fail.
( was bright red and looked like an F1 race car )
All good since 🙂
Follow the advice, you will find the culprit.
M.