Okay, I tried Paul and Si's ideas...
boot... remove "rhgb quiet" add "3" setup tool arrives - wait a bit and it goes again and I get a login prompt login as root
$ X -configure
nothing serious, says it can't find my mouse, lot's of stuff above screen I can't read.
$ startx
... and I'm back at where I started, the screen goes dead and I can see that 'crystals going to sleep' thing swish across the screen. The system is not powered down thou. Just no screen - maybe waiting with a login splash screen?
You can get some diagnostics on what's happening with startx as follows:
$ startx >X.out 2>X.err
Any console output generated by startx will go to X.out and any error messages will go to X.err. If it still hangs you can reboot and look in those files for any clues as to what's going wrong.
Paul.
