100Mbit/s LAN slow, TX only ~3MB/s (esp. file transfer) -- why?

[Luke]

On Wed, 14 Jul 2004, Andreas Ntaflos wrote:

> configured for full duplex and auto-negotiation, and actually running
> in 100baseTX-FD mode (5.1 uses rl0, 4.10 dc0, Gentoo and Windows are
> also equipped with RealTek RTL8139 NICs).

>From that statement, I'm almost certain the answer is "no", but are you 
using any ethernet-over-USB devices?
If so, you need to get USB 2.0 working throughout the system or you'll be 
limited to USB 1 speeds.
I really doubt this has anything to do with your problem, but it's the 
only thing similar that I've seen.