Weird network connection problem

[Yudong Tian]

Hi,
   We've got 200 nodes running,  with each node having a Biostar MB with a
built-in Realtek 8192
NIC chip, and running RH 7.2, 2.4-17. Recently we've seen a weird problem
with the
network connection to some of these nodes. About 20 of them will lose their
network connections
during the course of a few hours to a few days. And if I just unplug the
network cable from
such a node and plug it back in right away, the connection will come back
up.  Any insight
where we should look for the problem: hardware? BIOS? Driver  module? Such a
problem
does not happen to any nodes randomly. It only occurs to those 20 or so
particular nodes,
which are identical to others except their MAC addrs.

Thanks.
Yudong Tian