Problems after installing 1Gig NIC’s

We currently have one Win2K server and one NT4 server in a 10/100 switch (Buffalo). This switch also feeds 3 HP Procurve 10/100 switches.

Recently, we have replaced Buffalo switch with NetGear 8 port 10/100/1000mb switch and added 3Com 1Gigabit NIC cards into the 2 servers. We are yet to upgrade links from HP Procurve switches to 1gig links.

After installing NIC’s in servers, client PC’s (3Com 10/100 3C905B NIC cards) are hanging or loosing network drive mappings intermittently. There is no pattern to this (app related or otherwise).

Can anyone suggest what might be causing this.

Thanks in advance.