"ifconfig up" hanging on 6.1

[N.J. Thomas]

We have some Dell PowerEdge 1950 1u rack servers with GigE NICs on them
running FreeBSD 6.1. The NICs are connected to a Cisco 24 port 10/100
switch.

The interfaces are down when the programs that are running on them are
dormant, and whenever they receive a job, they bring the interface up.
(The other NIC is always up,  which is how the servers receive new job
notifications.)

A couple of times per day (about every hour or so on average), the
server receives a job, brings up the interface, and starts processing.

The thing is that occasionally, about 10% of the time, bringing up the
interface doesn't seem to do anything. It's as if the network is still
down. So we have to go any manually bring it up again for it to work.

We assumed that it was an auto-negotiation problem with the GigE card
and the switch, so we forced everything to 100Mbit, but it still occurs.

What could be the problem?

thanks,
Thomas