My internet connection is awful, and ping shows a fairly consistent pattern of timeouts - but I have no idea what to do to fix the problem.

I'm using wifi over an Airport Extreme basestation, and I've made sure that there's no channel overlap with other wifi networks in the area. The internet issues seem to be pretty consistent across devices (Macbooks, Windows laptops, iDevices, PS3), leading me to believe that it's an issue with either the basestation, the internet connection, or the modem - but I'm not sure either how to figure out which it is, or how to fix it.

I've tried using the ISP's DNS servers, as well as Google's, without much difference.

If I ping google.com, there's a fairly consistent pattern of timeout - long delay - gradual speedup - repeat. As an example:

64 bytes from 74.125.226.38: icmp_seq=50 ttl=57 time=37.314 ms
64 bytes from 74.125.226.38: icmp_seq=51 ttl=57 time=80.473 ms
Request timeout for icmp_seq 52
Request timeout for icmp_seq 53
64 bytes from 74.125.226.38: icmp_seq=52 ttl=57 time=2946.415 ms
64 bytes from 74.125.226.38: icmp_seq=53 ttl=57 time=1954.279 ms
64 bytes from 74.125.226.38: icmp_seq=55 ttl=57 time=248.792 ms
64 bytes from 74.125.226.38: icmp_seq=56 ttl=57 time=221.924 ms
64 bytes from 74.125.226.38: icmp_seq=57 ttl=57 time=358.521 ms
64 bytes from 74.125.226.38: icmp_seq=58 ttl=57 time=289.235 ms
Request timeout for icmp_seq 60
64 bytes from 74.125.226.38: icmp_seq=60 ttl=57 time=1809.925 ms
64 bytes from 74.125.226.38: icmp_seq=61 ttl=57 time=809.191 ms
64 bytes from 74.125.226.38: icmp_seq=62 ttl=57 time=425.607 ms
64 bytes from 74.125.226.38: icmp_seq=63 ttl=57 time=562.892 ms
64 bytes from 74.125.226.38: icmp_seq=64 ttl=57 time=212.179 ms
64 bytes from 74.125.226.38: icmp_seq=65 ttl=57 time=59.869 ms
64 bytes from 74.125.226.38: icmp_seq=66 ttl=57 time=120.611 ms
64 bytes from 74.125.226.38: icmp_seq=67 ttl=57 time=88.552 ms
Request timeout for icmp_seq 69
64 bytes from 74.125.226.38: icmp_seq=70 ttl=57 time=248.439 ms
64 bytes from 74.125.226.38: icmp_seq=71 ttl=57 time=222.373 ms
64 bytes from 74.125.226.38: icmp_seq=72 ttl=57 time=198.361 ms
64 bytes from 74.125.226.38: icmp_seq=73 ttl=57 time=34.753 ms
64 bytes from 74.125.226.38: icmp_seq=74 ttl=57 time=34.407 ms
64 bytes from 74.125.226.38: icmp_seq=75 ttl=57 time=61.647 ms

Speedtest.net gives me a 53 ms ping, a 2.62 Mbps download speed, and a 0.49 Mbps upload speed. My ISP provides me with 20 Mbps download and 2 Mbps upload, so I suspect the timeouts are interfering with the speedtest, or the speed is accurate and demonstrates how bad the issue is.

I've tried quite a few things, but I'm not sure which would apply - just let me know if there's any other information I could provide!