It occurred to me that Martijn van Buul wrote in gmane.os.netbsd.general:
> Just when i thought removing '-march-pentium4' solved everything, it went
> kaboom again. Right after I ran 'rtsol atu0'. Coincidence? Maybe, maybe
> not.

Disabling accepting router advertisements makes it seem solid - at least,
it's been up and running overnight, doing heavy NFS I/O, without any problems,
while it used to go belly-up in just a few minutes. So, it seems that my 
original problem is resolved - reducing the NFS blocksize helped. Thanks to
all who responded :)

Time to start up a new thread regarding the ipv6 issues.

Kind regards,

Martijn