Not directly
> o Is there a known bugfix for this?
No
> o Could it be a problem with ARP and/or proxy ARP? I wonder about
> the 'ping' effect?
It sounds like an ARP problem
> o If I can help tracing this down, tell me (how).
tcpdump will be a good start. Try and capture a stall then send me
the few packets before the stall, the stall and a bit after when it
recovers.