Intel PRO/100 NIC trouble when memory is more than 2GB with RedHat7.1beta

  I have a Mitsubishi electric manufactured Dual Itanium Box.
When the total memory size of the system is 2GB(512MB SDRAMx4), 
the PCI NIC intel PRO/100 card (eepro100) worked perfect with 
RedHat7.1beta.

  However, if the memory size is increased to 4GB, then with 
the RedHat7.1beta the behavior of the network become quite bad.

This can be verified by downloading a large file like 100MB or so 
by ftp connected to another neighbor machine across the 100Mbps SW-HUB.
The ftp stackes or very very slow down or network routing or ping dies etc ...
 In case of 2GB, the file can be transfered very quick. However with
the same configuration except the memory is 4GB it suddenly become lazy dog.

When I replaced the NIC to the older DEC 21140-AF based 100Mbps card,
for comparison purpose, then even with the 4GB of memory download works
file and fast and repeating the downloads does not make the network die at all.

I have to conclude that there is something wrong for the intel PRO/100
card suppord of RedHat7.1 when the memory size is more than 2GB.
If anyone give me a good pointer to fix this trouble, like by flush up 
some NIC card or the RedHat7.1 device driver version up etc., 
I thank you in advance.

  H.M.