Slow File Transfer Only If 10/100 Forced

Hello, and thanks to all who have previously posted. Many posts have helped me up until now.

My network has two machines that I'm currently working with. They are:

A) WinXP Pro SP3/Intel Pro 1000 CT Gigabit
B) WinXP Pro SP3/Marvell Yukon 88E8001
i) Network via WRT54GL Tomato (wired)

I have networked these machines across my LAN and achieved heinous throughput. I have manually set MTU/ACK/TTL etc. on both machines, as well as the router. Making those adjustments seemed to get me nowhere. At first I was stumped, but I went through every setting until I found one that "worked." Unfortunately, I have one issue that I cannot resolve.

As per my normal, I had forced all devices into 10/100 Full Duplex mode, and herein lies the quirk.

A= Intel Pro 1000 Gigabit
B= Marvell Yukon

If all devices are set to 10/100 Full, transfers from A to B are very slow (at most 200 KB/s), but transfers from B to A are nearly full capacity over a 10/100 Full line.

However, if I set A to Auto-Negotiate, my transfers from A to B run at full capacity (or close). B remains forced at 10/100.

Normally, I'd call this good and go with it, but when A is set to auto-neg. my normal internet connection loses 25% of its speed. Obviously, I have tried to get auto-neg. working at its best, but to no avail. It's a small thing to toggle auto-neg. when I want to transfer some files, but I'd love to know what's going on.

I've reached a point where I can no longer tweak any setting that I know of. I'm officially miffed. Can anyone offer a suggestion?