Crash when booting Ubuntu 7.10

When starting the system I have the following problem: The Ubuntu logo with the progress bar appears and then the screen goes blank instead of showing the login screen. The monitor switches off after a few seconds and the only way to get out of this is to hard reset my machine.
This does not happen at every boot. Sometimes the start-up process goes fine and the system is stable.

Some research in the log files in /var/log revealed the following line in kern.log:
Nov 29 20:32:03 compy kernel: [ 47.912175] NVRM: Xid (0001:00): 8, Channel 00000000

This line appears several times when the system fails to boot. During a successful boot this line is not in kern.log.

I have Ubuntu 7.10 (installed a short while ago). The graphic card is nVidia GeForce FX 5200. I installed the restricted NVIDIA driver.

Thanks for any help,
Ilan