Have you tried using a different router?
Check the ADSL filter - they can sometimes be the cause of intermittent quality of service issues.
Use a network cable and connect it directly to the router to rule-out the wireless functionality.
Change channels is good advice too, especially if there are a number of routers nearby.
Speak to your telephone company / ADSL provider and ask them to do a line check.
What wireless protocol are you using (on the router): 802.11a, 802.11b, 802.11b/g, 802.11g,
802.11n (150Mbps)
If you're using 802.11n - try changing it to 802.11b/g as your line may not support 150Mbps yet.
Cheers,
Shaun
