3com hubs losing connection

I have 24 port 3com hubs in three different locations on our WAN, all three locations have approx, 3o computers, some with 10 base T and some with 10/100 network cards. At all three locations the computers will lose the connection to the server. If we unplug the the power to the hubs and plug them back in after about 30 seconds they work fine for a couple of weeks, when it needs to be repeated. Any help on this problem would be appreciated.rstary@hotmail.com