This problem was solved by replacing the onboard Realtek 8100BL chip
to Intel's i82557 chipped card (Intel XPRES-PRO100).   Now I'm using this
Pen 4 1.6GHz + AOpen AX4B Pro-533 mother happily as our Auth/CPU
server for Release 3 Plan 9. It must be same for Release 4, I believe. :-)

Kenji