MonitorWare Knowledge Base

HI
I have a small LAN with 16 pc connected to a win2000 server through 2 switches.Only 2 pc-s have xp pro(each connected to a different switch) and the rest has win98.The problem is the following:
In the morning when we start all computers the network work fine for a few hours, but then i loose connection(can,t even ping)for 8 of the pc's, only works those connected to the same switch as the server.If i restart the pc's(win 98) they say no domain server available.It will only work again if i disconnect the 2 xp pc's from the network.The power suply of the switches is real can this make problems Surecom (10/100MBS )?
what the hell can it be?