I'm having this same problem, and I can't stand busybox anymore. I also got that nasty reboot cycle too when I changed /etc/passwd. Another thing that might work is to simply change the symlink from /bin/sh > busybox to /bin/sh -> /usr/bin/bash, which would be the cleanest way to change the shell. I'm too afraid to try this because I don't want to brick my 770 again.