2 Computers On Network Lose Server Access Randomly

Roughly 10 or so computers and 10 or so printers connected to a server 2003 machine with att dal internet. Upgraded to att uverse and because of our program that we use (hotel management software) we have to keep the gateway / ip's all the same. Well for whatever reason, just so happen that our specific info that we have to use, the uverse modem has reserved for its self and we cant use it on there. We hook up a cisco e1000 router and have it act as the gateway so that we can configure our settings the way we need. Get everything going just perfectly fine. Sometime that night, 2 of the computers lose server/networked files access, but still have internet access. They only lose access to shared files, printers, etc... Logging off/on or restarting the pc's fixes the problem. But then every night almost like clockwork around the same time frame, it all happens again. I've changed ip's on the machines, changed settings here and there, but nothing has fixed the problem. The biggest pain is, I cant replicate the problem, I have to waot until that night to know if what I did worked or not. The last thing I tried was changing the lease expire timer in the router from 24 hours to 7 days. I figured I'd give that a try since it seems that its almost 24 hours exactly between errors.