YDL 3.0/iMac DV SE - it works, but screen goes black after first successful graphical login

[Kristiaan Coppieters]

I've successfully installed a stock YDL 3.0 on my iMac DV SE (500 MHz G3
with 1GB of memory and 30GB hard disk).

Everything works fine for the first session - I can log in and use the
system just fine. 

But when I log out I don't get the graphical login back - instead my screen
goes black. 

I can still switch to the other consoles using Command-Option-F1,
Command-Option-F2,... and on the black screen, I can log back in 'blind'
(typing into the black screen) - the machine is still working fine.

It just looks like the X server not properly resetting the video hardware
for the graphical login - some case of 'bad reset'.

I've tried lots of things - for example, switch back to runlevel 3, then
back to 5, but that does not help - the only thing I can do is reboot the
machine. I've added various video= entries to my yaboot.conf. I've re-run
Xautoconfig. I've use the 'Display' system settings control panel (which
disables everything so I need to restore my configuration file - it tries to
use 'r128' instead of 'fbdev')

I am pretty sure it's some little thing somewhere in my X configuration, but
I have no clue as to what it might be.

Anybody seen something similar and found a solution?

Thanks!