I had a cordless phone system that was 900MHz (if I remember correctly).
Only thing I can think of is that cordless phones automatically switch channels in an attempt to find a clear one. They may have decided on one that is a harmonic of the radio transmitter frequency. When unplugged they may have defaulted to some other channel then after awhile gone back to that problem channel for some reason.
Kind of far fetched but possible.
-Hal
