Hm, thanks Darryl. The datasheet isn't really clear in this case (written in the datasheet is only PORTB<7:4>). I always thought portb change is the whole portb as the name says.
I'll check this out tomorrow. Hope it works.

That complicates my circuit a little 'cause I wanted the rb7 and rb6 only for ICSP. Now I have to solder in some jumpers :-)