Netgear switches dropping connections

My network consists of 25 PCs (win98/2000) and 1 windows 2000 server. I use Netgear FSM 726s switches (3). For some unknown reason the switches just loose all connections. If i reboot them and plug PCs/server in one at a time its fine but then within an hour the PCs drop out again. If i use old d-link hubs the network is ok, but this is not a viable solution. I phoned netgear and they swapped all the switches – but the problem has happened again! No PCs or the server have viruses. the firmware on the switches are up to date. Can anyone help????