Is dirty power the culprit?

These swings in performance are tied to the AC.

I always admire the way that vague symptoms in a nine line post (sounds "wonderful" versus "collection of items purchased at toys are us") can be diagnosed by others with such unquestioning certitude even though they haven't seen, much less heard, the system themselves.

It would like phoning a doctor who has never seen you, complaining of generic symptoms, and getting a precise diagnosis.

Power line issues can cause problems but it is hardly the only cause. Several avenues of investigation have been suggested to the OP. I hope he finds one - or a combination of them - helps with his problem.

While I am a firm believer that the listener is a major variable, and that often differences will be perceived when nothing whatsoever has changed (including the recording), I think that if the differences here are as great as you seem to be describing, something else is going on.

I'd suggest first that you buy or borrow a multimeter, and measure your line voltage on the good days and on the bad days. Also, take an AM portable radio, tune it to an unused frequency near the bottom of the band (e.g., 540kHz), set the volume level to a repeatable point, and compare how much rfi it picks up when placed near your power cords, on the good days and the bad days.

That will address (at least roughly) two of the four ways I can think of by which power might be affecting your system (voltage and noise). The other ways would be harmonic or other distortion, and dc offset, but you would need more specialized equipment to address those factors.

Beyond that, my bet would be (as Mapman suggested) that you have an intermittent problem in one of your components. Perhaps a capacitor that is leaky and on the verge of breaking down. Perhaps you can borrow some other component(s) to swap in on the bad days.

Regards,
-- Al

So if the problem is indeed dirty/variable quality power, aren't there meters or other gadgets out there that can measure and confirm this rather than guess or assume and possibly avoid a (potentially expensive) wild goose chase?

voltage variation, temperature in the room, tubes, oxidation--there are many variables.

i would consider some kind of power conditioning as has already been suggested. perhaps, you could borrow some device, before commiting to a purchase.

Maybe leave your system on for several days and see if you still experience the variation- especially if you have tubes in your system.

I hate to use the "C" word, but capacitors are strange little critters. A buddy of mine back in the 80's told me that they mechanically adjust themselves and can make a system sound dull until they have had time to form. Fact or myth, I know that if the amp/preamp has been turned off for several weeks the system will sound dull for a day or so.

But even back then, my buddy's tube amps would some nights sound dull and they attributed it to the caps.