Inconsistent 2.4ghz Signal from Dual Band Airport Extreme

I bought an Airport Extreme and Express last week and set up a wireless network with the Express extending the Extreme's network. Everything worked great for a day when, suddenly, all of my non-iPad portable devices stopped reliably connecting to the network. When they finally do connect, nothing will load; web pages will hang and hang and hang. I bought the WiFi Explorer app to see if I could figure out what was going on and discovered that the 2.4 Ghz signal from the Extreme is all over the place. It'll be strong (~80%) for a minute or two, and then suddenly dip to zero for several seconds. This happens over and over without a recognizable pattern (at least to me; see image). I've reset the cable modem and done hard resets of both the Extreme and Express. I've also reset the network settings on the iPod Touch and iPhones in the house, none of which has had an effect. Any suggestions?

Additional info, which may or may not be relevant: I originally bought the Express and that worked fine alone, but had to get the Extreme because the signal from the Express wasn't strong enough for my house. Of course, I only used the Express alone for a few hours. Also, I sometimes have trouble connecting to the wireless network when waking or restarting my iMac, but I get around that by turning wifi off and back on. I have a feeling I need to get another Express, but the iDevices don't work even when I'm literally standing over the Extreme.

Thanks in advance!