AE drops internet connectivity to Apple products

I have an Airport Extreme base station connected to a Motorola Surfvoard 6121 to connect to Comcast, and Airport Express extending my network. My wi-fi network drops internet to all Apple products on my network about every 3 hours, but PC remains connected. I have reset my AE, Express, Motorola Surfboard 6121, and all computers. The quick fix is to unplug AE every 3 hours and plug it back in, but I would like to fix the actual problem. Comcast says if I have any computers that can get internet access through my wi-fi then the issue is with the AE. AE user guide says to do a hard reset, which I have done several times with no success. I also have a 1-year old Airport Express which I use to extend my network. I have tried using it as the connection to the Motorola but it drops the connection to Apple products as well. I think it the issue must be between the AE and Motorola, but having reset both several times I'm at a loss for what to do next. I've read many fora and tried just about everything suggested. Any new help/ideas welcome.