Flashing rectangles across the screen

I am using Mac Mini (2013 Fusion) with a Samsung SyncMaster 2253LW monitor (2008). Every few days it shows flashing rectangles across the screen for a few seconds. Each rectangle is about ⅛-inch high and perhaps ¼ long with several in a row across the screen. I tried to capture it with a screen shot but that only captures the signal going to the computer, not the display itself so no problem was seen. I did a Samsung Monitor Self Test. No problem found, but since my problem is intermittent, is this conclusive? I can’t unplug the monitor fast enough to test it while the problem is occurring. I also replaced the DVI cable between the monitor and the computer.