I have tried some more things.
I thought maybe it is the: tv / stereo / lamp / tft screen.
I recorded a few minutes while turning all things on and off.
This does not make a difference.
I tried an other firewire card (dynalink, SB Audigy and onboard), does not make a difference.
I haven't tried an other cable, but I dont think it is the cable.
In the one second between two 'scenes' there is no audio, but also no noise.