Weird ifconfig issue

I setup a new server last night and we had some odd
issues transferring data to it from another server. 
It's a 5.2.1 RC2 server (it also happens on my server
at home).

the if driver is em0.  If i do the following command:

ifconfig em0 media auto
I get (between servers) up around 10MB/s

if i force the setting to:
ifconfig em0 media 100baseTX mediaopt full-duplex
(which is what it autosenses at) I get (max) 6Kb/s

Any reason why it would do this?  I've ran 4.x fine
forcing the media/media opt, but under 5.x i get
horrible speeds.

nice speeds:
em0:
flags=9843<UP,BROADCAST,RUNNING,SIMPLEX,LINK0,MULTICAST>
mtu 1500
        options=3<RXCSUM,TXCSUM>
        inet x.x.x.x netmask 0xffffff00 broadcast
x.x.x.255
        ether 00:30:48:28:ff:e2
        media: Ethernet autoselect (100baseTX
<full-duplex>)
        status: active

poor speeds:
em0:
flags=9843<UP,BROADCAST,RUNNING,SIMPLEX,LINK0,MULTICAST>
mtu 1500
        options=3<RXCSUM,TXCSUM>
        inet x.x.x.x netmask 0xffffff00 broadcast
x.x.x.255
        ether 00:30:48:28:ff:e2
        media: Ethernet 100baseTX <full-duplex>
        status: active


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