Upgrading 5.1 to latest -> page fault in kernel (crash) upon starting X

Hi,

Seems like I'm running into a serious problem upgrading my 5.1 machine 
to the latest level:

The symptoms: I did an upgrade (cvsup) then went through the usual 
build/install cycle (see below).

When I boot the new kernel everything runs straight up to
the moment when I start X-Windows. When starting X as a normal user I
end up with a "db"-prompt (debugger?) - from there I can only "reset" 
:-(. (for details about the messages see below) When starting X as root 
the screen goes blank and after some seconds the box reboots.

As far as building & installing is concerned I did everything as per
chapter 21 of "the handbook", including installing binaries (userland)
in single-user mode, running mergemaster etc.etc i.e.

make buildworld; make buildkernel; make installkernel
shutdown -r
fsck -p
mount -u /
mount -a -t ufs
swapon -a
cd /usr/src
make installworld


When booting the old kernel (kernel.old) everything is fine again -
so assume the problem is with the kernel itself. The "old" kernel is the 
one that has been installed when installing 5.1 from the CD.

I also did a "diff" between the GENERIC file that came with the original 
  installation and the one that was installed using cvsup (i.e. the one 
used in the fresh build) - no difference except the version string.

I tried configuring "device mgadrm" into the new kernel (this wasn't 
there in the the original GENERIC though) - to no extent - same crash.

Also: I've no indication about a problem in either /var/log/messages or 
the X-log - seems the box crashes before being able to write anything to 
the log.

Has anybody else seen this? What can I do against it?

Since I don't want to post exessive logs in the first place, here's what
I've got:

Asus CUSL2-mobo
512MB RAM
2 IDE HDs (80, 60GB)
Matrox G400 graphics

When upgrading I went from the version that comes on the 5.1-CD to
the latest one available via cvsup (the "UPDATING" file has an entry of
2003-09-28)

Please let me know if you need any additional details.

Thanks much in advance for your help,
-ewald

------------------------------< Cut here >-----------------------------
Here's the output from "db" which is where I end in when I start X:
(BTW - does anybody know a way how to capture this output besides 
"writing it down"?)

Fatal trap 12: page fault while in kernel mode
fault virtual address           = 0x1c
fault code: supervisor read, page not present
Instruction pointer     = 0x8: 0xc062e40e
Stack pointer           = 0x10: 0xdcaa0b68
framepointer            = 0x10: 0xdcaa0b68
code segment            = base 0x0, limit 0xfffff, type 0x1b
                         DPL 0, pres 1, def32 1, gran 1
procesor eflags         = interrupt enabled, resume, IOPL=0
current process         = 595 (XFree86)
kernel: type12 trap, code0
Stopped at _mtx_assert+0x4e: movl 0x1c(%ebx),%eax