>When I plugged in
>a network cable it reported 1000base-T 

So far so good.

>but when I ran dhclient it went
>back to 100base-T and then got an IP address. When I first ran dhclient,
>it sent out several requests and then by the fourth or fifth it
>apparently switched to 100base-T and got its IP address.

Hm. Just for diagnostic purposes, can you try assigning a static
ip address, configuring that address at boot, and not running dhclinet?
(The idea is to never take the interface down after power-up).
Then do a hard power cycle.

If the link camme up at 1000baseT, and dhclient ifconfig'ing it up and
down causes it to drop down to 100Base-TX, mabye there's a bug in
PHY reset after link down (or link up?).