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.