As for the old PC, sounds like overheating. Try de dusting. Make sure all fans spin, yada yada. If you just want it to run for another month, try an external fan with the side cover off.
My latest failed PC died for another reason but with simular symptoms. The power supply connector on the ASUS MB could not handle the 5V current, got warm so it corroded so the contacts (3) became worse, getting warmer, corroding more, until the voltage drop through the connector became too great. Try unpluging it and see if 2 or 3 pins look discolored. Pluging it back in might fix it temporarily.
