networking issues under heavy load

System: Sun Blade 150, gem0:

gem0 at pci0 dev 12 function 1: Sun Microsystems ERI Ethernet (rev. 0x01)
gem0: interrupting at ivec 3006
bmtphy0 at gem0 phy 1: BCM5221 10/100 media interface, rev. 4
gem0: Ethernet address xx:xx:xx:xx:xx:xx, 2KB RX fifo, 2KB TX fifo

nfs filesystem mounted using (nfs server being a netapp):

mount -3 192.168.1.201:/vol/vol1/scratch /scratch

Netbsd:

NetBSD x.x.x 7.0.1 NetBSD 7.0.1 (GENERIC.201605221355Z) sparc64

while running "rsync -avx / /scratch/" the system displayed in
/var/log/messages:

Aug 24 07:03:07 x /netbsd: nfs server 192.168.1.201:/vol/vol1/scratch:
not responding

Operations on /scratch are blocking afterwards, even a "umount -f
/scratch" (or umount -f -a -t nfs) never returns, while rpcinfo
192.168.1.201 displays RPC information accurately after the "non
responding" issue.

Any advice?