It is more than likely a bug in the rendering of the print image from the PCB file and nothing to do with the printer. I did a test board and it was the same. The error is present in the print preview image and any printer driver will just reproduce it.
After a bit more trial and error work, It looks like the parts (pads) objects are rendered before the others like tracks etc, and then made up as a composite.
I would suggest making the greber data, import it to Camtastic and then print a composite image from there.
This would be quicker than fising the Protel issue.
:R