During heavy traffic, I get the following errors logged a few times over a
short period, then the network connection, despite being "up" is not
responding and no traffic passes. This did not happen until a nightly
offsite backup was put in place over the link. There is no way to throttle
the traffic generated.

kernel: vr0: tx shutdown timeout
kernel: vr0: restarting

Running ifconfig vr0 down; ifconfig vr0 up from exec.php restores network
connectivity.

It appears to be a bug within FreeBSD which was possibly fixed with a
patch way back in v4.7, but the patch seems to have regressed in 1.3b15's
FreeBSD which this system is running.

Link to patch info:
http://ftp.kaist.ac.kr/geoul/pkgs/freebsd-cvsup/data/prefixes/FreeBSD-gnats.current/gnats/i386/44572
Link to bug info: http://www.freebsd.org/cgi/query-pr.cgi?pr=90890

Reports in the bug suggest that it only happens with half-duplex
connections. Unfortunately, of course, the kit it's connected to is only
half-duplex.

Does anyone have a similar experience?

Hardware is a CN700 chipset with a VT6102.