Hi Andrew, thanks for posting the file. So it looks like the main problem is that there are spots on the curve that have a series of control points stacked up right on top of each other. That's something that will tend to cause problems for 3D CAD use of the geometry.

If you select the curves in MoI and use "Edit > Show pts" on them there will be a red square displayed around each location with stacked up control points.

If I remember right I think there is a method built in to PDF/AI8 file import to try and clean those up so one thing you might try to start with is to save from CorelDraw as a PDF file and open that rather than DXF format.

Or actually even saving to PDF from MoI and then loading it back from the PDF file will do that, that's what I did to the attached file which should work ok now.

- Michael