There are so many reasons why this can happen it is hard to know where to start.
Was your system working previously and if so, do you know anything that has changed?
Some of the many reasons are
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.
Suggestions (not in any particular order)
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.