First post. Hi all from Spring, TX.
I have had this same problem...here is what I do:
Before sending the image to rip (I use the flexi rip/print), rasterize the transparent lens image and make sure "transparent" is checked. Important...make sure you don't have any "guide lines" on the lensed image or they will become part on the image. Choose what DPI you would prefer, and print. If any abnormalties show up in the print, go back (undo the original rasterize), repeat the same process, but choose a lower DPI this time. Sounds backwards I know, but the lower DPI will print a slightly more pixelated image, but no lines. Hope that makes sense!