network degradation

The signal strength of both my networks (i.e., 2.4Ghz and 5Ghz) has suddenly dropped. I use a Time Capsule and also use an AirPort Express to extend the 5Ghz network (TC upstairs and AE downstairs). The signal received by my client devices (e.g., Roku box) has dropped (e.g., from 5 bars or "excellent" to 2 bars or "poor"). Also, the client is now seeing two weak 5Ghz signals instead of one strong (extended) one as it did before. I've tried to troubleshoot this, but everything checked out ok, and I can't find any problem. Of course, it is possible that there is some sort of external interference, but everything was fine a couple days ago, and I think it unlikely that both frequencies would be affected (e.g., 2.4 would be affected by cordless phones, etc., but not 5). Any thoughts, ideas, solutions?
Thanks.