Actually, the problem seems to be localized around qe0 or the machine, which is attached to that interface.

I did a pr some time ago, but I managed to specify the problem.

If I upload data from the machine attached to qe0 with high speed, the sparc 
will freeze. However if I download something from the web to it, sparc acts as 
normal. This does not happen with the machine attached to qe1, even though the
NAT rules are the same.

Problem report on GNATS:
http://www.netbsd.org/cgi-bin/query-pr-single.pl?number=17956