Does your router allow you to lock the ip addresses of each connected device (so that those addresses remain permanent if a device is turned on and off)? If it does, then use that function.

I used to have that problem at random. I have up to 13 devices connected at the same time and found occasionally, for no apparent reason, 2 devices would have the same ip address. It didn't care until you visited certain sites, or tried to print on the wireless printer.