I meanwhile checked adaptive clocking with Gnome. No difference so far. The grka is running at maximum speed all the time.
I in addition tried the 270.26 Beta driver. No difference either.
Furthermore I tried reducing the clock frequencies manually by using "clock frequencies" in "nvidia settigs" (with the 260.19.44 driver running) and got a total freeze of the X11 system some seconds later when moving a window on the KDE screen. No visible reaction to mouse or key input any more after that. Had to restart the whole system.
Very similar to the instability problems reported at