Looks like you're right...
I also did some more testing with the loose setup today and only got into trouble when I touched the apron. Well, and as I actually don't plan to do this in a race, I can use my first setup.

(I updated it in the first posting.)

The main reason I tightened the setup for was that I got loose with it on a Papy server. I could catch the car without bringing anyone else into trouble, but these car control problems should not occur. I still don't know what it was caused by, well, I probably touched the apron in traffic and didn't recognize it.