Can I accept my own reply as a solution? It ended up being something stupid as usual. Somebody had checked the "scale lineweights" box in the pagesetup dialogue. Then I guess someone else unchecked the "print object lineweights" box when they ran into problems with plotting. Unchecking these boxes and being sure all geometry in the background was set to "By Layer" fixed the problem. And it only took an hour to figure that out.

Thanks for the suggestions Chris and pendean. Nice to see you, pendean. You're a legend, man.

My best,

Michael Ingram

Miller Company Landscape Architects