Since my last posting (Weird X freeze on Ubuntu 9.04, part umpteen), the problem with X freezing still seems to be present, despite the bug report saying it was fixed in development versions of Ubuntu 9.10. I upgraded to 9.10 on Friday, and have had a couple of freezes since then. Just now, I have just over 40h on the uptime meter. I think this is because I'm logging out before suspending the computer. Anyway, logging in is better than having freezes which require a reboot!
