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
Edit:
The following method from the linked 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.
