I have seen this on a Tru64 box with a Gbit card.  It was not resolved but
appeared to be a driver issue as the same throughput was observed on an ftp
test.  We exhausted the other network component issues to no avail.

I only add this as an experiential reference point to perhaps point to the
finger in another direction.  If somebody has AIX with Gbit working fine get
the driver/adapter rev/firmware levels from them and compare to yours.

Kelly J. Lipp
Storage Solutions Specialists, Inc.
PO Box 51313
Colorado Springs, CO 80949
lipp AT storsol DOT com or kelly.lipp AT storserver DOT com
www.storsol.com or www.storserver.com
(719)531-5926
Fax: (240)539-7175