Hi David, also the problem may be that your original curve is not quite 100% flat.

So for example this curve here:



If you go to the top view and zoom in to this area:



You can then see that there are some parts of the curve on slightly different levels:




That means that the extruded solids have a rather more complex intersection probably with some barely misaligned pieces than if you had a completely flat original generator piece.

You can flatten that piece by going to the top view and using the Edit frame to squish it down, there will be a "flat" snap that shows up. Seeing that flat snap is how I knew that it was not actually completely flat to begin with, because that snap is not present if it is already totally flattened.

- Michael