Bob, if you have a computer back-up power supply, try plugging the rig in to 
that, pulling the main power, sending CW, and seeing if the problem appears.
Hopefully not, and that isolates it in your house.
Then, turn on power a circuit at a time, see if you can replicate the 
problem.  I had something like that once, and it turned out to be the 
charger for one of those robotic vacuum cleaner things.  I don't know how 
generating RF effected the thing, but it cleaned up when I unplugged the 
thing and then hid it.
Where is the robotic vacuum honey?  My dear I have no idea at all!  I am 
blind don't use it and never knew we had it.  When did you buy that and not 
tell me about it?
Oddly enough, just recycling the house power has eliminated lots of problems 
that I was never able to find.