I have started to experience a problem whereby items connected to the wired port of my router suddenly loose connectivity

This starts by the fact that internal devices can no longer talk to each other, even though the router sees them on the network. The devices themselves can still connect to the WAN so they have not lost connectivity completely. However shortly after this happens connectivity to the wan will also be lost.
This can only be corrected by restarting the router. I have upgraded to the latest firmware since this started happening and refreshed the firmware, both leaving existing settings and a full factory reset removing all settings. I have also minimised all other settings so that now, with the exception of fixing the IP's of a couple of machines and forwarding a few ports, the router settings are almost virgin.
I have also tried to remove one or more of the machines but it makes no difference - still after 1-2 days, this degradation starts and the router needs to be reset.
Once reset all functions fine once more for a period.
I cannot identify any pattern that stops these working.

Does anyone have any ideas what I could look at?