Here is the 3dm again, without being swept. Easier to see what was swept. The helix was made in MOI, using the helix command.

Hi Nick. You will get much better results if you use 2 rails for the sweep. Just use Scale2D and make a copy of the helix a bit smaller or bigger than the original, snapping to helix center when you set the scaling origin.

jonah

Well I'll be darned. So what makes that work, more control points? Thanks!

All curves have their own normal direction, which is perpendicular to the curve along it's length but can be twisted in different ways. See this screenshot from Rhino. you can also see the helix's normals have a bit of curvature change near the ends. This is what probably caused the wrinkle in your surface. When you do a 1-rail sweep, the profile is attempting to follow the helix's normal direction in a literal way, because it has no more information to guide it. So if the helix normals twist in strange ways, so will your surface. Adding a second rail gives more info to help guide the sweep...

jonah