I have another data point that might be interesting for this discussion...

I can't make the NetBSD Tulip driver work well at 100 megabits in 
full duplex mode.  That's with a 21143 on an Alpha.  It just drops 
packets.  Not many of them, but enough to confuse things under heavy 
load.  telnet and ftp mostly work OK. 

But if you try to run a tcp throughput test like netperf, it drops 
a packet every time it gets up to speed, including on the retransmissions.  
Each retransmission attempt gets another packet or 6 across but then 
it drops one and goes through the long timeout path.  (Neeless to 
say, throughput isn't very impressive.) 

Half duplex mode works fine. 

I think the FreeBSD driver on an Intel box does the same thing.  
I think the drivers are very similar so that's probably not very 
surprising.

I haven't looked at the sources.  One of our guys spent a lot of 
time working on the FreeBSD driver and he didn't find anything. 
 
----

I also had some troubles at 10 megabits.  I've forgotten the details.  
It was the first time I tried to install a system so I was easily 
confused.  The problem went away when I switched to 100 megabits 
and I didn't go back to investigate. 

I do remember that the problem went away when I used ping on another 
machine to bounce an occasional packet off the machine I was working 
on. 

I think the symptom (without the ping running) was that nothing happened.  
Period.