white lines in large plot

Hi,

I have a plot with about 2 million points.  It should be a fairly dense 
plot over the reals, but I'm seeing many narrow, white vertical lines 
uniformly spread in my plot where nothing was plotted.   I'm using a 
period as the plot character.  I've played around with the cex 
parameter, but I can't eliminate the white lines without making the plot 
look too blocky. 

I'm using R version 2.4.1.

Thanks for any help,
Paul