Well, I spent the day looking at the way various applications read and write dxf files, as well as the differences between Acad versions.
My own Cad program outputs a dxf that is no longer compliant with Acad v 11 (maybe not even since 2000). But fortunately, it hasn't affected my work. I have been writing a SU plugin to modify my Cad2Dxf output in order to improve its performance. As long as I was doing this, I thought I might be able to post a DxfIn for SU-free users, but have discovered that there are too many considerations, requiring rewriting my code to accomplish even a very basic version of this. Good thing for me that my Cad2Dxf is easy to understand.
Thanks for the support everyone, hopefully I will finish soon:-)