surf command gives false lines

Hi,

I'm trying to plot using surf and I got the following weird result:

surf(peaks(40)) gives me this -> http://imgur.com/IM8Ga

If you look carefully at the two holes you will see a horizontal grid of black lines which should not be there. Also you can see a line that goes from (0,0,0) to (30,30,0)

When I plot the same figure using surf(peaks(40),'edgecolor','none') the figure appears exactly as it is supposed to. (see here http://imgur.com/md6wP)

Any idea??

Thank you

George

PS.: when I print the image e.g. using print -dpng name then they appear just right