have you tried assigning ip address to each device? by telling each device what address to use they will not ever have to ask the router for an address. your router should be addressed something like 192.168.0.1, or 10.0.0.1. if it is 192.168.0.1, device 1 would be assigned 192.168.0.2, second device would be 192.168.0.3, and so forth.
http://www.howtogeek.com/howto/19249/how-to-assign-a-static-ip-address-in-xp-vista-or-windows-7/
the only address that actually ever needs to change is the modem and that is done by your isp.
once you assign static ip addresses to your devices the problem should go away