I use runlevel 3 as standard but sometime I need a GUI.
How can I fix this? How can I find some errors or such. Can I uninstall KDE/Gnome and reinstall it and that might fix it?
No, reinstalling that stuff will not fix it. Did you try and rename/remove your /etc/X11/xorg.conf if it exists? If that doesn't fix it is there any output at all when you try and start it? Did you look at your /var/log/Xorg.0.log after trying to start X? What chipset?
Yeah, the new X shouldn't need xorg.conf where the old one did and the settings from the old one was probably screwing it up. If you want it to start in graphical mode just change the default runlevel from 3 to 5 in /etc/inittab.
That I can understand, but I still haven't made enough changes to a network card yet with and without dhcp, so I can't remember all settings.
and they keep changing too between new releases... well almost between everyone