rtl8139 problem

Hi,
I have 2 Linux Boxes connected through a twisted class 5 cable.
One has a RTL-8129 pci 10/100 card and the other has earlier a
NE2k 10 isa card and now has a newer RTL-8139 10/100 pci card.

Here is my problem: when the NE2k was connected I could tranfer files
from one PC to the other at the regular 10Mbps speed.
Now that the cards are both 10/100 and so they could reach the nice
speed of 100Mbps I get a much worse result: on the path 8129->8139
I reach at most 1.4Kb/s [measured through a ncftp download], while
on the opposite trajectory the trasfer speed does not pass .4Kb/s!!!

I tried with both modules 8139too and rtl8139 and the mii-tool command
tells me the "negotiated 100baseTx-FD, link ok" message for both.
How can it be possible that the cards negotiate a 100Mbps speed but then
behave so ridicolously bad?
Has anybody any solution to this problem? It would be very nice avoiding
buying a pair of 3com to reach 100Mbps...

Thanks,
 Roberto De Leo