Thank you for your help !
In /boot/grub/menu.lst I tried different vga settings, now changed to "vga=0x318"
I commented out the modelines, started X with "startx -- -logverbose 5" and created the attached log.
Now with "vga=0x318", when rebooting or powering ON right after the SuSE menu
have 100% of the screen, but it is text-based. It switches to normal blue SuSE_boot_image after runlevel 5 then final (GUI-)Desktop appears and works fine after that stage.
how do I get the normal blue SuSE_boot_image right from the beginning ?