You should verify that all your expansion cards and
connections are firmly seated inside the PC. Over the many times the
PC is booted and turned off, the temperature inside the PC cycles from
cold to hot and back again. These temperature changes causes the
metal on the chips, boards and plugs to slightly expand and
contract. This causes them to slowly move out of where they are
seated, which is called Chip Creep.
Many times when the system battery goes bad it can cause similar
symptoms. It is the small watch battery on the mother board.
If you haven't replaced it already, it should be replaced.