NFS writes over a VLAN trunk wedges system

I have a system running 5.1-RELEASE-p2 which is an NFS client of
another FreeBSD (4.x) machine.  When I have the NFS mount via a VLAN
the system reliably hangs (no response to console, including
Ctrl-Alt-Esc).  This is a default NFS mount (no options) and I am
trying to do a buildworld with /usr/{src,obj} NFS mounted.  The VLAN
is using an Intel fxp NIC.  The kernel has DEVICE_POLLING specified
but not enabled.

- NFS mount to the same server via normal Ethernet (using a 3Com 905
  NIC) successfully manages buildworld
- Adding WITNESS and INVARIANTS didn't help (though it lasted longer
  before dying)
- Enabling device polling didn't help
- Adding DISABLE_PSE allows it to complete a buildworld

Any suggestions on where to go next?

Peter