Ctrl-Alt-F1 plus subsequent Ctrl-Alt-F7 leads to black multiple screen(s) and a total freeze of PC f

I use the proprietary driver 352.30 with Opensuse 13.2, KDE 4.14. The graphics card is a GTX 750 TI with an ASUS 1920x1200 display at the DVI port and 2 Dell U2515H at full resolution (2560x1440) at the 2 HDMI ports.

xorg.conf entries are set such that all 3 screens are used when X11 starts for KDM. After a log in into the KDE plasma shell the xinerama configuration of nvidia-settings works perfectly for all 3 screens - as long as I stay on the graphical console tty7. No problems at all …

Switching to a text consoles via Ctrl-Alt-Fn from the graphical interface works. The text console appears always on the DVI screen - as grub and all boot messages do.

However, switching back via Ctrl-Alt-F7 leads very often and irregularly to 3 black screens and a total freeze of the PC. Not always but lets say in 80% or 90% of the trials. If black screens appear after Ctrl-Alt-F7 the PC does not react to mouse, keyboard and remote ssh connections are no longer possible. Segmentation fault ??? Kernel problems ???

The freeze almost always occurs if one of the two HDMI screens goes into power saving mode - but even then sometimes (actually very seldomly) the switch back to the Plasma shell works. From power LEDs one can see that the system tries to start all screens - but they turn black.

Very often also logging in into the text console (tty1 - tty6) and doing something there triggers the freeze for Ctrl-Alt-F7. But no real systematics …

I could not find any special relation to graphical applications started on KDE, either.

HOWEVER, and remarkably, such a problem does never (!) occur when X11 displays the KDM login screen (and a graphical background pattern on the other screens). From the graphical KDM display I can switch with Ctrl-Alt-Fn to whatever text console I want, work there how long I want and then switch back to the graphical console again. No problem. It does not matter then whether one or several of the HDMI screens goes/go into power saving mode. Switching back to X11 (KDM) starts all screens again and X11 displays KDM + background on all screens.

It seems that the driver works together with X11 as intended; but not with the KDE Plasma shell.

Any ideas , suggestions ? What could I do to narrow down the error ? The systemd journal just indicates the reboot - no specific error message there …