You are going to have to dig around and determine which device is cause the issue.
First make sure you have the latest firmware on your router.
Its pretty standard troubleshooting. Start at the cable modem and work out.
Plug directly into the modem with your mac.
If it fails its more than likely the ISP or the modem but be sure to try another pc/mac because the ISP will blame your device until you prove it is not.
Plug in the router and plug your mac into the router with a cable.. ie no wireless.
If it fails you have a router issue if it works then all that is left is the wireless.
If you end up with the wireless being the issue you will need to test your mac on another router to determine if its the router or the mac that has the issue.
I have a new linksys WRT310n router and a brand new Motorola surfboard modem. I use my mac laptop and i have everything hooked correctly and all the lights on the modem and router that are supposed to be on are always on. My laptop says that i have 100% signal and am ALWAYS connected even when the internet is not working. When the internet wants to work it works really good but then it just drops all of a sudden and doesn't work sometimes like it has a mind of its own. I have it set up in a extra bedroom so theres no interference with cordless phones and other electronics. I have tried calling my ISP (which is cox) and they cant help because they keep telling me nothing is wrong on there tests because everything shows that its working fine. Please help, I don't mind paying 50$ a month for internet if it works but it never has worked right since i got it months ago