are you exporting lines, closed lines, faces, or polyfaces? Can you "tesselate" your parametric objects? Hope you're not using any acis callls. Maya's dxf support is a bit weak in comparison to other software. get rid of any parametric modelling (I assume you can do this upon exporting from acad...i never used it so i have no clue what exporting out of acad is like.
If you have formZ there, try importing your acad dxf into formZ, and export it as an obj with unix lines and parametrics as solids/faces. I would think that there would be some sort of plugin/script/autocan freeware converter that could spit out a simple obj...?
-Don
