newnfsd's stuck on "ufs"

[rick at snowhite.cis.uoguelph.ca]

I'm working away at porting my NFSv4 server to FreeBSD5.2. It goes along
ok for a while, but when doing several ops concurrently, most of the nfsd
threads end up stuck sleeping on "ufs" as shown by the attached "ps axl".
(The amusing part is that, once all but one thread is stuck, the last
 thread works fine. In other words, the "fix" is to only run one newnfsd:-)

Anybody happen to know off the top of head, what I've screwed up?

Thanks for any hints, rick
--- ps axl of newnfsd ---
    0   523   522   0   4  0  1192  736 nfsd   I     ??    0:05.34 newnfsd: ser
    0   524   522   0  -4  0  1192  736 ufs    D     ??    0:00.02 newnfsd: ser
    0   525   522   0  -4  0  1192  736 ufs    D     ??    0:00.02 newnfsd: ser
    0   526   522   0  -4  0  1192  736 ufs    D     ??    0:00.02 newnfsd: ser
    0   527   522   0  -4  0  1192  736 ufs    D     ??    0:00.02 newnfsd: ser
    0   528   522   0  -4  0  1192  736 ufs    D     ??    0:00.02 newnfsd: ser
    0   529   522   0  -4  0  1192  736 ufs    D     ??    0:00.02 newnfsd: ser
    0   530   522   0  -4  0  1192  736 ufs    D     ??    0:00.02 newnfsd: ser
    0   531   522   0  -4  0  1192  736 ufs    D     ??    0:00.02 newnfsd: ser
    0   532   522   0   4  0  1192  736 nfsd   I     ??    0:00.01 newnfsd: ser