Hi Mike, unfortunately it's really difficult to help you very effectively without seeing your objects. You don't need to post an entire model, just the one surface that you are trimming and the one cutting curve would do.

If you want to keep it confidential you can e-mail them to me at moi@moi3d.com.

If you can't send anything at all, then the main thing you'd want to do is closely examine the cutting curve, zoom in to them very closely and look for anomalies. Turn on control points and pull them around and look at how it deforms, if there is any "back tracking" with the curve coming back over itself (which makes it self intersecting). Look at the object type indicator in the upper right corner of the window when the curve is selected, does it read "Closed crv" or just plain "crv". Try squishing it down in the top or side view and see if it gets a "flat" snap or not - if it does not then it's not actually planar. Those are some of the various things I would do myself to investigate the cutting curve to see if it is well formed or not. There are possibly some other kinds of diagnostic methods that I'm not thinking of right now off the top of my head, it's much more normal for someone to post the geometry to get help and then I examine the geometry closely.

Also it's pretty rare but occasionally some kinds of display problems are actually just bugs in the display mechanism itself and not problems with the geometry. One way to test for that is to do a test export to a polygon mesh format like OBJ and see if it looks different in the export mesher than it does in the modeling display.

- Michael