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There are many potential sources of interference in the 2.4 GHz band used by 802.11b/g/n, including cordless phones, Bluetooth, microwave ovens, and neighboring WLANs. Techniques to identify and reduce interference depend on the source.
The most common source is co-channel interference from neighboring WLANs (including competition between your own two routers). When you configure a Wi-Fi access point (or router) to use a specified channel, the channel is centered within a frequency range that's about 20 MHz wide. For example, when you select Channel 6 (2.437 GHz), your access point (AP) actually transmits between 2.427 GHz (the center of Channel 4) and 2.447 GHz (the center of Channel 8). To avoid co-channel interference, adjacent Wi-Fi APs must be configured to use non-overlapping channels. Most people use Channels 1, 6 and 11 because that results in the max three non-overlapping channels.
You can determine whether this is your problem by using a shareware tool like NetStumbler to detect nearby APs and the channels they are currently using. If your APs are not using two different channels in {1, 6 or 11} then reconfigure them to do so. If other nearby APs with strong signal are using one of these three channels, then avoid that channel. For example, if one of your routers is close to an AP on Channel 9, 10 or 11, then pick Channel 6 or 1 for that router. With only three non-overlapping channels, it can be hard to find "clean air" so just look for the best possible channels and of course avoid overlap between your own routers.
If Wi-Fi co-channel interference does not look to be your problem, then start looking for another kind of wireless device causing interference. Begin by eliminating obvious sources. For example, if you use Bluetooth peripherals, try moving them at least 30 feet away from your routers, avoiding the likely transmission path between your routers and clients. If you notice that interference gets worse when you turn on a microwave oven, try setting your router to use Channel 1 (the channel farthest from the RF range generated by microwave ovens).
If you don't spot any obvious sources, you can dig deeper using a wireless spectrum analyzer. To learn more about troubleshooting Wi-Fi interference using spectrum analysis, read this searchNetworking tip: WLAN troubleshooting with spectrum analyzers.
Most spectrum analyzers require buying a specialized wireless adapter. However, if you happen to have a relatively new laptop containing an Intel Centrino 2 wireless adapter, you might want to try AirMedic, currently available as a free beta trial.
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