Existem muitas razões pelas quais isso pode acontecer, é difícil saber por onde começar.
Seu sistema estava funcionando anteriormente e, em caso afirmativo, você sabe de alguma coisa que mudou?
Algumas das muitas razões são
Faulty router.
Interference from another Wireless router.
Too much noise on the telephone line (from within your house)
Problem with the telephone line (outside you).
Problem with your internet provider.
Bandwidth being used up.
Problems caused by virus.
Sugestões (não em qualquer ordem específica)
Switch off the router for a while (Maybe even overnight).
If possible, try connecting via a cable rather than wirelessly.
Ask your ISP to investigate your line quality.
Temporarily disconnect all other equipment from your telephone line.
Connect your modem to the master telephone socket.
Move your router away from another other electrical cables/equipment
Scan for virus/spyware
Your wireless router should have an option to change channels which
can help avoid interference from other people's equipment.
Try a different browser.
If you have access to a good internet connection, download and burn a
Linux LiveCD which you can boot off.
Try another router.
Check your wireless security to make sure no-one is hijacking your line.