Fatal Trap 12

Hi,

last year I install FreeBSD 5.1. I updated kernel and userland regularely
and had no problems.
Two weeks ago I upgraded to CURRENT and the troubles began.
After Kernel updates ( last I downloaded at 24.2.2004 ) I
got 

  Fatal trap 12: page fault while in kernel mode
	...
  supervisor read page not present
  current process ( swi:8 tty:sio clock )

This trap shows the same process every time and occures
some seconds after 1 hour after booting ( uptime about 1 hr 10 seconds ).
What can be the reason for this ? 
Only once the system runs longer than this hour,  but after 1 week I
shutdown the system and at next boot I still had the same probem.

I read about things like that in the mailing list. Is it still an open
problem ? 

I wanted to build a debug kernel, but had problems because it 
takes me more than 1hour.

 Thanks , Lutz