News: I actually can switch power levels as much as I like as long as X was not
started yet. Starting X afterwards gives me a stable system with changed power
level. As soon as it was started once however, the lockups happen on trying to
change the power level. Even if I shut down the X server.

Downgrading xorg-x11-server from 7.6_1.16.99.9-2.2 to 7.6_1.16.1-350.4 fixes
this problem but then I get random lockups and crashes which is not much better
:/