Shimmering/interlaced visual glitch

I am seeing shimmering interlaced sort of display of shadows frequently. See below:

http://dl.dropbox.com/u/7990437/flickering.png
http://dl.dropbox.com/u/7990437/ue4_flickering.mov

Mac Pro 2013 FirePro D700 6GB OS X 10.9.2/10.9.3b

ok - it turns out that I had accidentally copied the plane over itself so it is Z-fighting. I deleted the duplicate mesh and it’s gone.