TARYNTOO  Signal and noise drop to zero?

I've noticed in the graph view (only 100 samples so far) that several access points that are being polled frequently drop to zero signal and noise, often for only one or two polls, but occasionally for a dozen or more polls. My Airport 'frisbee' never seems to do this, but my airport express and a neighbor's linksys drop regularly.

Is this an issue with AirRadar, my internal Wifi (Broadcom chipset in an aluminum iMac), the access points, or just a power saving mode on those routers?

I noticed that while viewing a graph, set to show 500 samples, the window went black, the graphs disappeared. Switching back to the scanner view, I saw that several of the access points were gone, I suppose because they'd been at zero beyond a time threshold. Very disconcerting behavior, and it appeared that I'd lost all those prior samples.  
(Version 1.1.7)