problem connecting gigabit and fast ethernet?

I had a server with a DLink DFE-530TX+ 10/100 ethernet card connected to a Cisco
DSL modem/router and a Compaq Presario laptop with built-in 10/100 ethernet via
a Netgear dual-speed hub. Everything was running at 100 megabits according to
the lights on the hub, and everything was running smoothly, although I never got
anywhere near 100 megabits because neither the laptop nor the modem could
provide that sort of throughput. The laptop would do significantly higher than
10 megabits on large files from the server, however. I replaced the system with
the DLink card with a system with an ASUS P4P800 motherboard with a built-in
gigabit ethernet interface from 3Com. The first thing I found was that telnet
connections from the laptop to the server were unusable -- there was a
tremendous lag, and frequent disconnects. The next thing I found was that
transferring large files from the server to the laptop via SMB directory sharing
resulted in the speed rapidly oscillating from far below 10 megabits to far
above ten megabits, accompanied by the collision light on the hub being on at
least 50% of the time. The resulting transfer speed to the laptop was lousy,
and made watching streaming video from the server impossible. (It was smooth
before, even though the speed didn't get much above 10 megabits at times because
the laptop is no match for the server.)

I'm not sure that this is a hardware problem because while I was installing
drivers for the new hardware on the server I decided to do a full Linux upgrade.
But transfers go smoothly with small files, I can print from the laptop to the
printer hosted by the server, I can telnet from the server to itself, and the
server talks to the modem with no errors or reduced speed. It's only when
filling the pipe from the server to the laptop that things get weird, other than
that telnet problem. The reason I'm bringing it up on a hardware group is
because the new motherboard in the server has developed another (unrelated)
problem, and if I have to replace it I'm wondering if I should get something
different. (I was going to try the old network card on the new board, if it was
still running.)