All of the above, plus maybe the day-to-day voltage swings from your incoming power to your dwelling. See about getting a plug-in voltage meter (not very expensive I think) and examine readings the next several times to see if there's any variation that seems to reliably correspond to your listening involvement issues. If so, you can then think about doing some homework with various solutions like power conditioning or voltage regulation or even power regeneration and the like. There should be at least some kind of level of solution for your particular budget. This sort of thing I describe is not uncommon and is usually referred to as the "time-of-day" effect, but it can vary on a day-to-day basis as well. Hope this helps.
Regards. John
