What's in these files? Mainly text, lots of pictures, or a mix? How many pages usually. Are they simple files or really complex with lots of things going on?
Right now my suspicion would be on fonts (usually a major culprit), but I don't know enough yet. Something to try would be appending the layout. Make a document of the same size as these documents (or larger) and append the layout of the older documents to your new document.
No guarantee that will work. It may just hang there too.