Just to get on the same page (sorry for the pun), can you tell us explicitly which driver you're using? I assume it's the Brother (vendor-supplied) drivers? Does this problem only occur when printing particular documents, or from a particular application? If you print to file first (to create a pdf document), then print that file does this issue still occur?
Does printing a simple pdf document from a terminal window produce a good copy as expected?
Code:
lpr -P <printer_name> /path/to/foo.pdf