In one of the computers that is regularly affected, open a Command Prompt window ("Start -> Run", type CMD and click OK) then, at the prompt, type...
...and press Enter. Look for the time that the "Lease Expires". If it is expiring around 4pm on the current day, that may be the problem. The Lease should normally be renewed automatically by the DHCP server (Router) long before it reaches its expiry time, but this may not be happening correctly. If this is the case, the solution would be to increase the Lease time in the router's settings to something like a week.
You could, of course, give all of the computers in the network fixed IP addresses.