Ubuntu 8.04 in VMs under VMWare Server 2.0.1 need the kernel parameter "clocksource=acpi_pm" when they boot. Edit /boot/grub/menu.lst and add "clocksource=acpi_pm" to the end of the line that starts "# kopt=". Don't remove the "#". In this context it's not a comment. After you save the file, run "sudo update-grub" and then reboot.
If the guest doesn't have this kernel parameter specified, time runs backwards on the guest O/S, or time hangs or gets stuck. This was showing up especially on my backup server running bacula, and the backup clients that had large amounts of data. I suspect that high loads exacerbate the problem.