X-Server fails to start

[Klaus-Peter Schrage]

Every once and a while, say in 5 out of 10 boot events, the boot process 
stalls and ends up in a text console
(kernel 3.11.4-201.fc19.x86_64, KDE 4.11.2-1, NVIDIA graphics card with 
nouveau driver):

[snip]
          Starting The KDE login manager...
[  OK  ] Started The KDE login manager.
          Starting Command Scheduler...
[  OK  ] Started Command Scheduler.
          Starting Job spooling tools...
[  OK  ] Started Job spooling tools.
          Starting Wait for Plymouth Boot Screen to Quit...
[  OK  ] Started Machine Check Exception Logging Daemon.
[  OK  ] Started Install ABRT coredump hook.
[  OK  ] Started LSB: VirtualBox Linux kernel module.
          Starting LSB: VirtualBox web service API...
          Starting LSB: VirtualBox autostart service...
          Starting LSB: VirtualBox balloon control daemon...
[  OK  ] Started LSB: VirtualBox web service API.
[  OK  ] Started LSB: VirtualBox balloon control daemon.
[  OK  ] Started LSB: VirtualBox autostart service.
[  OK  ] Started IPv6 firewall with ip6tables.
[  OK  ] Started IPv4 firewall with iptables.
[  OK  ] Started Login Service.
[  OK  ] Started Network Manager Script Dispatcher Service.
[  OK  ] Started LSB: DKMS kernel modules installer service.
[FAILED] Failed to start Wait for Plymouth Boot Screen to Quit.
See 'systemctl status plymouth-quit-wait.service' for details.
          Starting Manage, Install and Generate Color Profiles...
[  OK  ] Started Avahi mDNS/DNS-SD Stack.
[end of output]

Sometimes there are a few more lines of output after FAILED, but the 
boot process always finishes around that line.

The last lines of Xorg.0.log are:

[snip]
[    45.416] (II) NOUVEAU(0): NVLeaveVT is called.
[    45.417] (II) NOUVEAU(0): Closed GPU channel 0
[    45.447] (EE) Server terminated successfully (0). Closing log file.
[/snip]

I can get to another text console, but doing 'startx' only gives me a 
very rudimentary graphical desktop, not KDE.

I have searched bugzilla for the term 'Failed to start Wait for Plymouth 
Boot Screen to Quit' with no results, and even googling didn't produce 
anything to help me further.

Any help of the wise people on this list would be greatly appreciated.
KP