Network Troubles

I have a peer to peer network of 13 PCs. Periodically, (once or twice a month), the network traffic all but stops. Some of the PCs can be seen on the network, but the majority cannot. Networked printers cannot be reached. There have not been any recent changes to the network.

Our ISP provides a router for internet traffic, from there we have a Linksys router for the firewall, then a switch.

If I power down the switch, then turn it on, everything returns to normal. I don’t see any abnormal IP traffic in the Wireshark captures I’ve taken.

Any advice or clues as to where I should be looking would be greatly appreciated.