-CURRENT spontaneously rebooting when using X

Could someone advise how I can get a bit more information to report?  :)

I've just installed a fresh 6.0-RELEASE and immediately upgraded it to 
-CURRENT (most recent yesterday evening) and installed the latest xorg 
servers.  When I start X, usually within a few seconds to a couple of 
minutes the system falls over, but unfortauntely it doesn't leave me at 
the kernel debugger and doesn't do a kernel dump for some reason or other, 
it just goes back to the BIOS and reboots.  Can anyone donate a clue to me 
for what I need to do to find out what's going wrong?  I'm using the amd64 
compilation, if that makes any difference, and I've had the same problems 
with both the generic configuration as well as my customised one.

I've experienced one or two other panics when not using X, but I was left 
in the debugger on those occasions so I don't know if they're related or 
not (I saved the kernel dump from one of them but have since overwritten 
the kernel, so it's probably not terribly helpful; not doing too well so 
far, am I!)

Cheers,

Chris.