I get this message (like every few minutes):

tlp0: receive error: runt frame

which suggests that there's short frames, and the switch I'm hooked up
to agrees. I assume it's because of the card, and not the link. I have
366 undersize frames in (at least, I think there is a bug in the
uptime counter in the switch) 633 hours.

Worth investigating, and if so how?

tlp0 at cardbus1 dev 0 function 0: Xircom X3201-3 Ethernet, pass 0.3
tlp0: interrupting at 11
tlp0: Ethernet address 00:04:ac:51:32:20
tqphy0 at tlp0 phy 0: 78Q2120 10/100 media interface, rev. 10
tqphy0: 10baseT, 10baseT-FDX, 100baseTX, 100baseTX-FDX, auto

/Johan