Ok, it stopped when I plugged out the controller since I used the internale BEC. ;)
http://cx4.de/rc/XS6r1.jpg
http://cx4.de/rc/XS6r2.jpg
I opened up the receiver and found lots of dirt.
I tried to get most of it out.
At first it didn't seem like anything changed.
Then it didn't act weird and now I got the receiver mounted on the servo again and everything seem to be ok again..
That's quite mysterious. ;) Hope that it doesn't change it's mind all the time now.