Your backtrace suggests that X is being killed with a SIGQUIT signal. We had this problem earlier in the Lucid development cycle, and it was caused by an interaction between X and plymouth.
I notice you've got a non-standard “console=tty1” option on your kernel command line. Does this behaviour persist if you use the just the standard “quiet splash” kernel command line options?
Your backtrace suggests that X is being killed with a SIGQUIT signal. We had this problem earlier in the Lucid development cycle, and it was caused by an interaction between X and plymouth.
I notice you've got a non-standard “console=tty1” option on your kernel command line. Does this behaviour persist if you use the just the standard “quiet splash” kernel command line options?