Checksum Errors on GigE Cards

About 18 months ago we upgraded all of our key servers to use fiber optic
gigabit ethernet cards. We went with Compaq NC6136 cards attached to an
Extreme Networks Summit 48 gigabit ethernet switch, and several servers are
attached to Cisco 2948 switches with gigabit ethernet modules added. To
my horror, in chasing down some other network issue, I have seen today on a
sniffer that every one of the machines with these cards is giving repeated
checksum errors on TCP connections. I verified carefully that the only
interfaces giving checksum errors are the gigE cards, and all of our 100
megabit over copper interfaces are working perfectly.

Is there some trick to configuring settings on a gigabit ethernet card under
Windows 2000, or alternately to making these cards work with a fiber optic
card on an ethernet switch? I had thought that this would be a pure plug
and play operation, and that fiber optic gigE would reduce latency at very
least. A few bad connections I could possibly believe might be caused by
dirty fiber cables, but because every one of six interfaces is exhibiting
the same defect, I am more inclined to believe that we have messed up on
some configuration issue for the cards under Windows 2000, or alternately
some settings on the switch.

Any advice on what might be causing these checksum errors is appreciated.

--
Will
westes AT earthbroadcast.com