It will most likely take a while for us to find out what exactly is causing this, as so far it has been impossible to reproduce this here. Any example documents which lead to this error are apprectiate, if you can supply any please send them to support(at)pdfforge.org (we won't be able to use the autocad file though).
I have the same issue.
I send you (to your support) a file (Voiture autonome 0.pdf) that occurs this bug.
This pdf is the result of several merged pdf files. As you can notice, all the pages are not in the same format.
Then I use PdfCreator to fit all the pages in the same format (A4). When I launch the pdf creator impress, the bug appears at the end of the process.
Hope this sample help you to fix this anoying bug.
Ps : my configuration : Windows 7 pro 64 bits / Ghostscript 9.20 for Windows (64 bit) / PDFCreator-2_4_1