Ok, here is the deal, I have to do this all the time and in the beginning it frustrated me to no end. Autocad though vector based is not able to handle true b-splies. Though they import and you can see them fine, they list out as splines, etc. they are still a proxy object. Rolling over them with autosnaps on will send your computer into brinlock every time. It will usally come back out of it but it takes so long you stop caring, and in the meantime your processor(s) are maxed.
The solution is to export the file to a DXF file ver. 12 or below. The reason is that prior to Autocad 13, the spline objects were defined as line segments instread of splines. The problem is that Illustrator only exports to ver 13 as I recall so I'm not sure how you can do this other than exporting the file out of autocad to a ver 12 file. I use Coreldraw 12 which has a direct export to ver 11 and it has worked flawlessly since I first found all this out.
Hope this helps