Stanley/Belkin Power distribution extension which I think is to prevent surges
kapt,
It's a long shot, but you could try plugging the PC into the mains directly just in case it improves things (thus implicating the extension socket).
In my experience, extension sockets are often poorly made. I recently found one with a 'dead' socket because all of the internal connection screws were loose, and a while ago I had one that was completely missing an earth terminal on one of its sockets.
That apart, I have to say that at the signal levels Kapt is running at (>=63dB attenuation), it takes only a tiny amount of interference signal to register as 1 or 2 dB on the SNR. At that attenuations, some small 'drift' in SNR, as various bits of stuff are switched on & off, is going to be almost innevitable and perhaps impractical to totally eradicate.
