It would be interesting to see on what wchans processes are waiting (ps -alx).
I see similar behaviour on ibmnws with pcn nic. In my case client usually
sleeps in getput and nfsd in nfsmblk, that means that nfs_writerpc waits
for packet being sent, and client waits for nfsd to complete. I also see pcn reset
by watchdog before hung and it seems pcn loses mbuf pointer without freeing it
during reset.

	Andrey