I’ve been having a lot of luck with this tool http://kabeja.sourceforge.net/ (don’t let looks deceive you 
).
It works for me where most tools produce garbage results. Also Inkscape is notoriously bad with DXF. Both import and export. I’ve found Illustrator is much better, but it’s not free.
While we’re on the subject, here’s a tip: what I recently started doing when working with different vector softwares is to include a line of predetermined length, say 10mm, in my design. Then when I import that file in a different application, I’ll make sure that line is 10mm. If it’s not, I’ll scale everything in the file to the correct size (whatever % needed to get the line to 10mm). If I have something else in my file that’s easy to reference, I’ll use that instead of the line. This makes sure that the design ‘survives’ the conversions happening between the different appilcations, which seem to have trouble conforming to file formats.