I had some trouble with some Illustrator curves. For example, I import a word (outlined) and some of the curves extrude in Z and other in -Z. So all curves extrude in the same direction, you can reverse the problematic curves in Illustrator like this:

http://graphicdesign.stackexchange.com/questions/53198/is-there-a-way-to-reverse-the-order-of-anchors-in-a-path [graphicdesign.stackexchange.com]