Word table border problems after pdf'ing

Sorry if this has been brought up before. What can be done about the faint lines that appear in some Word tables that have been pdf'ed? I say some because the problem only occurs in merged, shaded cells. Half the vertical lines of a cell are normal and half are noticeably thinner and print that way. If shading is removed before pdf'ing, there is no problem. Does anybody have a workaround for this other than not shading table rows? I'm currently using Acrobat 9 Pro Extended but it has happened in older versions too. thanks.