PDC dropping connections

I have a Proliant 2500 as a PDC in a domain that consists of a BDC and two Terminal servers. All the servers are connected to backup. The server are connected to 3com switches that are also powered through a UPS.
My problem is that once a week my PDC logs an event that the network link is down (it has an onboard cpqnf card).There is a 4 second time lapse between the link going down and the connection being reconnected. I have run diag on the card,and everthing is OK, I have also checked and changed the cabling. Any thoughts or suggestions would be greatly appreciated.
Thanks,
JoeB