Those are commonly called "jail bars" and are a symptom of faulty electrolytic capacitors.
Usually, the fix is to simply replace all the electrolytics in the monitor in one go, since they're so cheap, and most of the work is taking the monitor apart anyway. You can even buy a little kit containing all the capacitors you need for a particular monitor. It's called, imaginatively enough, a "cap kit", and costs about 8 bucks. Or you can simply buy the capacitors individually from any electronic supplier, that's even cheaper (or at least it is when you buy in bulk).
What kind of monitor is it?
-Ian