well... try to get the oops message out; for many causes of crash that's a good way to find out what happened...Unfortunately, I couldn't get the oops message. Because when this behavior happens, the machine hardly freezes and the SysRq combinations do not work. Also, I checked the logs (message,syslog,kern.log etc.) to see something suspicious. Unfortunately, there was none. Just one thing I noticed when investigating on the logs: the kernel is doing its regular jobs. Like cron etc. So, the kernel is not really dieing. Next time, I will try to ssh to my machine to check what's going on, when this behavior occurred.
(not always, but it allows us to find patterns if nothing else)