Try Restarting (Reboot) your system, if problem is still coming and going, then use antivirus software, in Safemode.
At least I think they are. My monitor was working fine this morning, but when I was downloading something and came back after a while, my monitor was displaying horizontal bars throughtout the monitor. They are kind of gray, but they don't display all the time. The desktop usually doesn't show anything, but once I open a file or a browser, they show up.