This might sound off the wall but trust me this has happened to me.
You say it happens "now and then" ? So it isn't always like this?
What else do you have plugged in to the same outlet (or along the same circuit as) your computer is plugged into?
Reason I ask is that one day I was busy computing and there was a outlet in the kitchen I was needing to repair, well I plugged the microwave into the same wall as the computer (not the same outlet but along the same line) while the microwave was running my screen looked just like that, this will also happen to a TV when a hairdryer is running
