Have you tried connecting it directly to the router with a wire to see if the problem is with the wireless?
If it works okay directly connected to the router then it's probably the wireless. Any number of things can interfere with wireless: a nearby router on the same channel or with the same SSID, older wireless phones, radio interference in general. Try changing the channel on the router and assign it a unique SSID. If it's an older router it may need a firmware update. Update the drivers on your WiFi NIC.
Hope this helps...