There is an improvement after a dist-upgrade a week ago. Now kernel 4.7.0-1-686-pae is running and since one week there are no freezes at boot anymore.
But with xfce running the system often freezed after a while. It never happened at the console. I found here: https://bugzilla.kernel.org/show_bug.cgi?id=109051 the script from Wolfgang M. Reimer (Comment 437), c6off+c7on.sh, and even after hours the system is running without freezes.
