Bizarre packet loss

[Gregory Seidman <gsslist+debian@anthropohedron.net>]

First off, here's the network architecture:

ISP (untrusted) router (NAT)
 |
OpenWRT (trusted) router (NAT) ... wireless device
 |
Cisco (unmanaged) switch-----------+
 |                                 |
Netgear (unmanaged) switch        wired device
 |
Debian server

If I ping the Debian server from OpenWRT, the wireless device, or the wired
device I get 25%-30% packet loss *unless* I am pinging an external IP from
the Debian server at the same time, at which point the packet loss goes
below 1%. I have no explanation for this.

But wait, it's even weirder than that. Pinging from one wireless device to
another wireless device *also* shows significant (~20%) packet loss unless
the Debian server is pinging an external IP. Note that pinging an external
IP from a wired or wireless device doesn't seem to have any impact.

I suspect that something bad is happening at the Ethernet layer that I'm
just not seeing, but I don't know where it's coming from or what I'm
looking for. I've used tcpdump from the places where I can (OpenWRT, Debian
server, wired and wireless devices) and looked at it in Wireshark, but nothing
jumps out at me.

There is a pretty strong chance that this isn't coming from the Debian
server at all, but a ping from it has some magical effect on the network.
I've been fighting this for days and I'm just at a loss. I'd very much
appreciate any advice or options to try.

Thanks,
--Gregory