iMac - Screen/ graphics card mystery?

Here's a riddle for you...

I've just got my new refurbed iMac... occasionally, and randomly, whilst loading my software and transferring files a red dotted line appears around the shadow of all windows, and across all toolbars.

When I restart the red lines dissapear.

I have taken screenshots of the problem, and when I open them whilst the problem still persists (and zoom etc.) the red dotted lines are still there in the screen shot. However, when I view the screenshot when the problem isn't there, or on another computer, the red lines are gone from the screenshot.

What is wrong with my mac? The screen? The graphics card?...

Thanks!