spontaneous reboot with large packet flows on net5501+lan1741

So I just bought a lan1741 for my six-year-old net5501 (running the
latest stable Linux kernel), because four ports just isn't *enough*.
Unfortunately, it seems to not work entirely well.

Enough packet flow over any ports on the lan1741 (not necessarily tiny
packets either: a few thousand 1500-byte packets a second are enough)
causes the net5501 to spontaneously reboot within a couple of minutes.
The kernel never gets a look in, as far as I can see: the reset is
immediate, nothing is logged, nothing is asserted.

There is no such problem with the built-in ports (as long as the flow
doesn't also involve a port on the lan1741), so my initial suspicion
that the thing might be running out of power seems to be disproved. As
it is, I have four ports on this machine that I can use only for things
that might just about fill a 10MiB/s network or maybe even less (I
haven't looked for the boundaries of this behaviour, though I might get
out iperf and try). VoIP works on it, but anything heftier, like wifi,
tends to cause reboots as soon as someone tries to, say, run a backup or
do an apt-get or a pile of NFS traffic over that interface.

So... has anyone seen this sort of thing before? Is this a known problem
with the lan1741?

-- 
NULL && (void)