Do you have another monitor or video card from one of the other computers you've replaced? Worth a shot. I had a monitor go south on me about a year ago. It didn't look exactly the same, but would do some funky thinks with the image. I could do the same thing, turn it off, and wait a minute, then back on, and it would work for awhile. You might see if just turning off the monitor without turning the computer off does the same thing. If so, I'd say monitor. If not, I'd go to the video card first. Hope this helps! Jim.
Home of Irish Setter Rescue of North Texas
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At a minimum, I'm Pentatoxic...but most likely, I'm a Pentaholic. There seems to be no known cure. Pentatonix, winners of The Sing Off, season 3