realtek 8139b response is very poor

Hi,

I have been using linux 2.4.2 kernal and have onboard ethernet realtek 
8139b chip. Its talking to vxworks system over a 100mbps line. There 
are only 3 pcs conneted in the network and I was try to get a 
consistent response between all of them. I always have problems with 
the linux system where I have occationaly ping delays are high and some 
time the system response is slow. Regulary I see ping response of 
240usecs and once in a while it jumps to 1.5 to 2msecs. I am trying to 
optimise the network to as to get very predictable response. Currently 
the network is running on half duplex. Will full duplex work. How do I 
set it up. I checked the settings using mii diag. I also see rx errors 
when ever I see these delays. I changed the rx and wx buffers on 
vxworks is 8k, I set the buffer on linux to 4k to optimise, but did not 
help.
I have the 8139too driver running. I think the rev is 0.9.?

Thanks in advance for the help,

Ravi