What Causes a White Horizontal Line across my Monitor After Quitting Blender?

I have an Intel IMac, two years old, with Radeon Video Card, 512 megs of VRam, and 4 Gigs of Ram. After working for several hours with Blender I noticed a thin horizontal line across the lower third of my 24 inch monitor. I rebooted the machine and the line was gone and it’s gone today, although I am leery of using Blender again because I know there is a video issue here, or a video card issue which has not occurred before using this application.

My question is: is there any way I could use Blender, perhaps change some user settings, so that it doesn’t tax my system?

I use Final Cut Studio, Shake and many other apps, but this is the first 3D app I’ve tried and I really liked it. It’s a shame to have to abandon it.

Any suggestions would be greatly appreciated.

By the way, I wasn’t aware of the line while working in Blender because my desktop was covered with the pdf file of the Blender book I purchased as well as a new file that I was working on. But, I haven’t had this line working with any other app.