Hi armin, I'm trying to split up my answer into some different parts.

You also wrote:

> If you look and the center model, there are quite some bumps
> on the left hand side at the transition to the straight
> shot. the same on the right hand side, though not quite as bad.
> The screen shots don't really show it too well.

Looks like some kind of wiggle in the surface interpolation where there is a sudden shift between your curved arc piece and the straight line segment which has no curvature.

One way to avoid it is by applying sweep a few different times, instead of doing the sweep all in one pass.

So for example, build this sweep first:



Then select this edge at the end:



And do another sweep using that:



And so on, that's how I made the attached version which seems to be a bit better than your original.


Also though your line is not exactly tangent to those 2 arc pieces, it is off by almost 1 degree on one side and about 0.5 degrees on the other side, that is probably not helping things out either.

- Michael