network traffic

[Buck Jones]

any one know were I can get a netwrk testing tool that can sit on a
server and test the speed of a network connection.. I have a small
network ot work and I get computers that just disappear off the
net..different computer at different times. but most of the time they
are on the net and just are so slow. I have check for virus's and adware
and changed the switching hub out..I have even replaced every network
cable in the place....  
 
 
I would like two programs that sit on two computer and just talk to each
other and tell what the speed they are talking and if there is a packet
loss