em0, VLAN and bpf(?) trouble w/RELENG_5

Dear FreeBSD developers,

I'we run unto a big problem with RELENG_5, built at Thu Nov  4 15:07:44
2004.

Hardware is P-IV 3.0 (HTT off in BIOS), Intel S875WP1E motherboard, Promise
SATA RAID onboard (1+0 setup, works fine), and 2 onboard LANs: em0 and fxp0.

em0 is in 802.11q VLAN trunk mode, connected to Cisco Catalyst 2950T. vlan0
is configured on top of em0 and acts as an Internet connection of the box,
it has a /30 IPv4 address block ("mini-ether") configured (2 addresses used,
one is at vlan0, other is Cisco router on the other side).

The problem. From time to time, vlan0 stops passing packets at all. At this
moments, Catalyst stops seeing MAC of vlan0 (it's the same MAC as em0) in
the mantione VLAN (untagged VLAN is also configured at em0 and works fine!).
This means that `show mac-address-table vlan XX' command on Catalyst don't
show the MAC.

The problem can be easily repeated manually. It's enough just to issue a
command like `trafshow -I vlan0' of `tcpdump -I vlan0' and voila! vlan0 is
out of business, no packets are going through.

System is Ok at the same time, no panic, no freeze, just no packets through
vlan0 that's it. Rebooting the box cures the problem (for some amount of
time).

Am I doing something wrong? Or is this a bug somewhere? In 4.* vlans worked
without a glitch...

Thanks,
Andrew