TCP segment of a reassembled PDU

One of my servers gives the continuous message "TCP segment of a reassembled
PDU" in a sniffer trace of its conversations with other machines on the same
ethernet segment. Some brief research suggested that there may be a
problem with the TCP MTU size on either end.

As a test, on one of the Windows 2000 workstations that has performance
problems with this server, I hardcoded through the registry the DWORD MTU in
the adapter TCP/IP Properties, and then set this to decimal 1400 (default is
1500 for ethernet). The workstation has a copper GigE adapter. The
server has an optical GigE adapter.

We rebooted the Windows 2000 workstation and instantly performance was great
with the server. Opening a message on that server took 4.5 seconds before
the MTU change now took 1/2 second after the MTU change. So clearly MTU
setting of 1400 did something good here. However:

1) The sniffer trace still shows the reassembled PDU messages.

2) The workstation immediately had severe problems using the file server and
was not able to use the Internet at all.

I backed out the MTU change and all resumed to its "normal" state.

Are there other settings that must be changed when you lower the MTU setting
on Windows 2000? What about Windows XP/Windows 2003?

What are some possible causes of these symptoms. Ultimately I would like
to know what changes would:

1) Make conversations with the server fast.

2) Remove fragmentation suggested by the reassembled PDU sniffer messages.

3) Allow normal communication through routers and onto the Internet.

--
Will