Well it looks like you're on PHP5 already - 5.0.4. Granted, I'd prefer it if you could upgrade to the latest, or 5.2.5 if possible. (I think that's still the most recent stable build).

You might also look in your /etc/log/error_log files to see if there is anything causing the issue showing up in the logs. I would also considering asking the guy who is the Admin of the server if there is another server you can test on, whether it has a similar architecture or not. Anything you can use to compare results, especially since so many of us can have an install without any issue. If this issue happened the majority of the time then I'd say it's a bug in Ubercart, but since it doesn't, I'd say it's not