Mysterious hangs

Hi,

For some time I've been seeing infrequent and seemingly random hangs on my
dual opteron system. Motherboard is Tyan K8S Pro. Until now I thought it was
a "hard" hang, but I've got serial console hooked up and BREAK_TO_DEBUGGER,
DDB and KDB in the kernel, and when sending a break through telnet I get:

telnet> send brk  
KDB: enter: Line break on console

And nothing more. Which means the box is sort of alive, but fails to present
even the kernel debugger. The machine is currently on 5.4 as of yesterday.
It's been hanging before (with anything from days to weeks between), under
various variations of 5.x, but I haven't had the serial console before now.

Has any one seen this before? Any ideas what I could look at? I suspected
hardware, but it seems that isn't the case as most HW errors I've seen would
cause a total freeze.

Here's to hoping.

Thanks,
/Eirik