Thanks for the suggestion. That does give me a way to check profiles for closure. I tried this with my part, which did turn out to have a small open spot, but when I fixed this, I still have the problem. The part looks good, and when cut with a simple square extrusion, all looks correct, however after I cut the part to put internal threads in the center, If I again do any operations to the outside, it opens up areas which let you see the inside surfaces, as if the part was just a hollow shell, with missing surfaces.
Any additional suggestions much appreciated.
I've attached a file, showing the part appearing as just a shell. You can even see "through" it.