The suggestion to use `Antialiasing->False` doesn't really solve the problem. I don't have a single solution that's appropriate in all cases, but I think one of the approaches I list on the following web page will work: 
[Avoiding artifacts in shaded contour and density plots][1]

**Edit:**

The following method from the lined article solves the problem:

Instead of exporting the image (assumed to be stored in `im1`), export the modified graphics

    im1 /. {EdgeForm[], r_?(MemberQ[{RGBColor, Hue, CMYKColor, GrayLevel}, Head[#]] &), i___} :> {EdgeForm[r], r, i}]

This replaces the invisible edges of the polygons (`EdgeForm[]`) in your graphic (called `im1` here) by edges of default thickness and with a color matching at least one of the neighboring polygons. The new edges then help fill any empty space between the shaded areas.

  [1]: http://pages.uoregon.edu/noeckel/MathematicaGraphics.html#ExportShading