The system was just started a few minutes before and I also put the "echo bfq ..." into /etc/init.d/rcS and tried it after several reboots. I was able to start xterm but as soon as I started something like cutetube or the browser it always crashed pretty fast. I also once noticed that it rebooted while the screen was turned off and the system was idling. So I guess that it still has something to do with swapping.

Thanks for the effort nonetheless!