Which one is it?
Have you set up Client IP filters with schedule rules to turn off internet access at certain times? Could be you have a dodgy ADSL line? Are your cable connections/adsl filter securely attached? Several things to check.
If you think it is def the router:
Assuming you have the modem setup webpage click here saved as a favourite in your web browser, open it and save your modem settings.
I'd then reset the modem and restore it to factory settings and start from square one again. If it works with basic settings, it's either something you've set up, or not the fault of the router.
If you are also using Internet Explorer, click tools>internet options>advanced>restore defaults to make sure your browser is set up for basic connectivity.
Has your ISP got a reputation for dodgy servers and connection problems!!