Hi Cube, yes can you please post the 3DM file? It's a lot easier to see what's going on by examining a 3DM file since it's then possible to look at the actual geometry an zoom in, look at different angles, etc... which can't be easily done just by looking at a screenshot alone.

When you get a weird looking result that has chunks missing from it or "leaking out" like that, it usually means that there is some problem with the trimming boundaries of that particular surface piece, they're probably mangled in some way like having pieces cross over themselves or things like that. That can be possibly the result of a bug, or it can also be the result of bad geometry inputs like having some original curves with little loops in them or similar stuff.

But it can be difficult to build a good quality model going in a "patch by patch" method like you're doing here because if you just build different surfaces next to one another they will usually not match up smoothly between each other and there will be a sharp crease in your model there.

Usually to get a good smooth model you would need to use filleting or blend which are the main construction tools that ensure smoothness of the stuff that they generate to the surrounding surfaces.

EDIT: sorry was writing this while you posted your reply above and so didn't see it.

- Michael