Running into image issues with firmware higher than 5.02.9B830

I discovered a strange and inexplicable issue (at least for me).
Always when I am trying to use a higher iOS firmware than 5.02.9B390 the image appears very fidget and unstable in a kind of an unpredictable judder effect while watching a movie via infuse.

With 5.02.9B390 everything runs buttery smooth and stable. In the best Steve Jobs way I could just avoid it to upgrade but I have to upgrade the firmware when I like to use streaming client apps like Netflix or Hulu here in Germany.

I am running several external hard disks connected to an Airport Extreme (the flat one).
What do I miss to make it smooth ?

Every advise is highly appreciated.

Regards from Germany