Random Disconnecting Internet. Would love some help.

I use a wireless router and cable modem for my internet connection. I use a laptop and my brother uses a PS3. When we use them both at the same time (him playing online multiplayer and me just loading webpages) the internet will cut out for me and him and cut right back in. When its only one person using the internet it will never cut out. But when somebody ELSE starts doing the smallest things on the internet it will sometimes cut out and reconnect 20 seconds later. Very annoying.

Connection Details:
Router: Cisco Linksys E2500
Cable Modem: Motorola Surfboard SB5120
Network Speed: 20Mb down/4Mb Up
ISP: Comcast

My network speeds are enough to support well over 2 devices but these random disconnects every few minutes is a big pain for me. I've tried restarting my router and cable modem and using Ethernet connections don't work either. This could be a router/cable modem issue, but i don't know for sure. If help is there i am in definite need.

Computer Specs:
OS Version: Microsoft Windows 7 Home Premium, Service Pack 1, 64 bit
Processor: Intel(R) Core(TM) i5-2430M CPU @ 2.40GHz, Intel64 Family 6 Model 42 Stepping 7
Processor Count: 4
RAM: 3947 Mb
Graphics Card: Intel(R) HD Graphics Family, 1845 Mb
Hard Drives: C: Total - 593993 MB, Free - 31310 MB;
Motherboard: Acer, JE70_HR
Antivirus: avast! Antivirus, Updated and Enabled