My first thought was "Are they old Compaq cards?"

I've run into problems before with Compaq branded network cards made back
in 1998-99.  If you stuck 2 identical cards in the same system it would do
funny things.  I think it was some early implementation of Compaq's
failover software that was trying to take over the cards or something
because it would arbitrarily switch IP addresses on the cards while the
server was running.  Sometimes one would stop working altogether and then
start up again on its own.  A real problem when we were trying to put one
card on an isolated LAN.  Finally, we switched one out for a 3COM, and
everything was fine.

Don't know if that helps or not, your mileage may vary.

- Kevin