Robert, thanks again.
Naturalness of links is given (to a large extent), internal structure as you recommend, and semantic clarity should be given (top down from generic to specific terms).
I wonder why this effect I speak about came up recently. Rankings were stable before for years, and I've changed nothing in the last weeks or months. What I observe seems to have nothing to do with penguine AFAIK because it's some kind of process, no sudden fall-back in rankings.
One aspect might be the content of my category pages. I have not altered this content for years as the pages ranked well. But my content was and still is copied (in parts, at least) all the time by some small competitors, Amazon and Ebay vendors - so I begin to think that maybe it's a Panda effect. But this content is mine, and I always thought that this was obvious for Google.
What do you think?
And if so, what to do? - I know that I have to act with care.