Time for me to get a new monitor?

[wiredspider]

This is my parents machine, it has a 15 inch compaq monitor. It's about 3 years old and sometimes the screen will go black/dark. You can see the very outages of whatever you were looking at. Anyways, to get it back to normal you have to hit the monitor. My dad said it needs be cleaned, but of course he knows nothing about computers so he wouldn't know that there are capacitors inside that could kill you. What would most likey cause the problem they are having and should I get them to replace it?