1. Your system is entering the
ddb(4) kernel debugger upon panic. When X is in use, the panic cannot be seen, and the system appears to "freeze". You can type ddb commands, however you will have to do so blindly.
2. The
crash(8) man page will help, also.
Setting the sysctl ddb.panic=0 will prevent the system from entering ddb(4) when a panic occurs. This is helpful for X users.
If you want to find out why the problem is occuring, the system will need a suffuciently large swap space to dump RAM to in the event of a panic, and your system will need sufficient freespace to store the dump from swap into /var/crash after RAM is saved to swap and the system reboot.
Since you can reproduce this panic at will, I recommend doing so once ddb.panic=0, and then following the guidance in crash(8) to obtain a backtrace and a list of the active processes, so that you can provide a full report the problem (with your dmesg) to the Project on the misc@ mailing list.