A problem about the network

[jbm3072]

HI，all.
   I have encountered  a problem about hardware .  I build up a hadoop cluster 
with  four old computer. After start the cluster and running job for  few days 
, I found the computer's network become slow,and the job's process time become 
longer and longer. It becomes normal after rebooting the computer.
   I asked the system manager,they said  the  network card may be not so good. 
   have you encounter the problem like this? and How to solve this question?
   Thanks.