Can't adjust Lombard's network duplex or speed

I have a maddening problem with my Powerbook G3 Lombard (bronze
keyboard) that I'm hoping someone can solve.

There's some kind of duplex negotiation problem occurring between my
Powerbook and the rest of my network in OS X 10.3.2 that doesn't occur
if I boot back into OS 9. I can transfer files down to the Powerbook at
top speeds, but when I start any transfer *from* the Mac, the speed
drops to almost nothing. (I've replaced the router, cables, and even
inserted a 10/100 hub between the Powerbook and everything else, with no
changes.)

This occurs no matter what method is used to tranfer files, including
NFS, FTP, and CIFS (Windows shares).

The solution *should* be to force the Powerbook from the full-duplex
that it's autoselecting to half-duplex, or even to drop the speed from
100 to 10, just to see what happens. That's where the problems
continue...

If I enter the following command as root:

ifconfig en0 media 10baseT/UTP

(or any other media choice shown by ifconfig)

I get this error:

ifconfig: SIOCSIFMEDIA: Operation not supported

If I adjust the speed or duplex settings through the new 10.3x network
configuration, my settings are ignored without any errors.

Interestingly, entering "ifconfig en0 mediaopt half-duplex" is accepted
without errors, but has no effect.

In case you're wondering, ifconfig -a shows the following:

lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
inet6 ::1 prefixlen 128
inet6 fe80::1 prefixlen 64 scopeid 0x1
inet 127.0.0.1 netmask 0xff000000
gif0: flags=8010<POINTOPOINT,MULTICAST> mtu 1280
stf0: flags=0<> mtu 1280
en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
inet6 fe80::250:e4ff:fe3a:d14f prefixlen 64 scopeid 0x4
inet 10.19.0.6 netmask 0xffffff00 broadcast 10.19.0.255
ether 00:50:e4:3a:d1:4f
media: autoselect (100baseTX <full-duplex>) status: active
supported media: none autoselect 10baseT/UTP <half-duplex>
10baseT/UTP <full-duplex> 100baseTX <half-duplex> 100baseTX <full-duplex>

Any suggestions?

Thanks,
Mike

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