Im assuming your running windows?
open command prompt and type "tracert www.google.com"
the first hop will be your router, the 2nd will usually be the computer at your local telephone exchange.
You can do extended ping tests against these 2 ip's to see if the prob is between you and your router (wireless),
or you and the exchange, (your phone line).
if the latency is high or you get loss at the first hop follow RealBeast's advice. use inssider to chose a different wireless channel.
you can do a more in depth test on each hop by typing "ping -n 100 -l 1000 [ip address of your router or whatever device /hop you are testing to]" look for dropping packets or latency spikes