If it is a desktop I would also say it is overheating.
Over time the fans and air vents clog up with dust and fluff, and the hot air cannot be extracted so the box overheats.
Take the main PC unit outside, remove the cover, and blow out any dust or dirt with a hair dryer (turn it on to cool setting).
If any dust or fluff sticks to a fan or heat sink GENTLY remove it with a piece of wood (not metal) or a small clean paint brush.
If you remove any dust / fluff blow it out with the hair dryer.
Do all this CAREFULLY so you dont dislodge any wires or components.
Take the system unit back in (but leave the covers off) and wire it all back up and turn it on and make sure all the fans are still working.
If everything seems OK put the covers back on.