Mysterious murder of gdm.pid

[livingroom]

Hi, another newbie here attempting to install YDL 2.1 on a PMac 7500
upgraded to G3 300.

My install seems to be successful but when it reboots and starts up I get a
problem when it gets to the log-in screen.  The log-in option just flashes
for a second and then I get a repeated flashing error message of some kind
which is something similar to this:-

According to /var/run/gdm.pid
gdm was already running but seems to have been murdered mysteriously

There is also a different (port?) number in there somewhere but it is all
flashing so quickly it's difficult to see it correctly.

I have tried ticking the no video driver option in bootX but ha just gave me
a totally dead monitor.

Any suggestions gratefully received but do remember I am a complete beginner
in regards to Linux.

TIA

Sie