After 5 years of warming up and cooling down, it's possible that some of the cards and chips have become a bit loose.
Switch off the pc, open the case and press all the cards down into their slots GENTLY.
Blow off any dust (there will probably be LOTS).
Next, press down GENTLY on the tops of any chips on the motherboard.
Also, check that the cable connecting the monitor to the pc is tight at both ends.